Fabrication of multi-layer dispersion-engineered waveguides

ABSTRACT

A multi-layer laterally-confined dispersion-engineered optical waveguide may include one multi-layer reflector stack for guiding an optical mode along a surface thereof, or may include two multi-layer reflector stacks with a core therebetween for guiding an optical mode along the core. Dispersive properties of such multi-layer waveguides enable modal-index-matching between low-index optical fibers and/or waveguides and high-index integrated optical components and efficient transfer of optical signal power therebetween. Integrated optical devices incorporating such multi-layer waveguides may therefore exhibit low (&lt;3 dB) insertion losses. Incorporation of an active layer (electro-optic, electro-absorptive, non-linear-optical) into such waveguides enables active control of optical loss and/or modal index with relatively low-voltage/low-intensity control signals. Integrated optical devices incorporating such waveguides may therefore exhibit relatively low drive signal requirements.

BENEFIT CLAIMS TO RELATED APPLICATIONS

This application is a divisional application of prior-filed co-pendingU.S. non-provisional application Ser. No. 10/037,966 entitled“Multi-layer dispersion-engineered waveguides and resonators” filed Dec.21, 2001 in the names of Oskar J. Painter, David W. Vernooy, and KerryJ. Vahala, which in turn claims benefit of: i) U.S. provisionalApplication No. 60/257,218 entitled “Waveguides and resonators forintegrated optical devices and methods of fabrication and use thereof”,filed Dec. 21, 2000 in the name of Oskar J. Painter; ii) U.S.provisional Application No. 60/257,248 entitled “Modulators for resonantoptical power control devices and methods of fabrication and usethereof”, filed Dec. 21, 2000 in the names of Oskar J. Painter, Kerry J.Vahala, Peter C. Sercel, and Guido Hunziker; and iii) U.S. provisionalApplication No. 60/301,519 entitled “Waveguide-fiber Mach-Zenderinterferometer and methods of fabrication and use thereof”, filed Jun.27, 2001 in the names of Oskar J. Painter, David W. Vernooy, and KerryJ. Vahala. Each of said application Ser. Nos. 10/037,966, 60/257,218,60/257,248, and 60/301,519 is hereby incorporated by reference in itsentirety as if fully set forth herein.

GOVERNMENT RIGHTS

The U.S. Government may have limited rights in this application pursuantto DARPA Contract No. N00014-00-3-0023.

BACKGROUND

The field of the present invention relates to devices for modulating,routing and/or processing optical signal power transmission. Inparticular, optical waveguides and resonators for integrated opticaldevices, as well as methods of fabrication and use thereof, aredisclosed herein. The waveguides and resonators include a multi-layerlaterally-confined dispersion-engineered waveguide segment, and mayfurther include one or more active layers, thereby enabling tailoring ofoptical properties of the waveguide/resonator, and/or controlledmodulation thereof.

This application is related to subject matter disclosed in:

-   -   A1) U.S. provisional Application No. 60/111,484 entitled “An        all-fiber-optic modulator” filed Dec. 7, 1998 in the names of        Kerry J. Vahala and Amnon Yariv, said provisional application        being hereby incorporated by reference in its entirety as if        fully set forth herein;    -   A2) U.S. application Ser. No. 09/454,719 entitled “Resonant        optical wave power control devices and methods” filed Dec. 7,        1999 in the names of Kerry J. Vahala and Amnon Yariv, said        application being hereby incorporated by reference in its        entirety as if fully set forth herein;    -   A3) U.S. provisional Application No. 60/108,358 entitled “Dual        tapered fiber-microsphere coupler” filed Nov. 13, 1998 in the        names of Kerry J. Vahala and Ming Cai, said provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein;    -   A4) U.S. application Ser. No. 09/440,311 entitled “Resonator        fiber bi-directional coupler” filed Nov. 12, 1999 in the names        of Kerry J. Vahala, Ming Cai, and Guido Hunziker, said        application being hereby incorporated by reference in its        entirety as if fully set forth herein;    -   A5) U.S. provisional Application No. 60/183,499 entitled        “Resonant optical power control devices and methods of        fabrication thereof” filed Feb. 17, 2000 in the names of        Peter C. Sercel and Kerry J. Vahala, said provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein;    -   A6) U.S. provisional Application No. 60/226,147 entitled        “Fiber-optic waveguides for evanescent optical coupling and        methods of fabrication and use thereof”, filed Aug. 18, 2000 in        the names of Peter C. Sercel, Guido Hunziker, and Robert B. Lee,        said provisional application being hereby incorporated by        reference in its entirety as if fully set forth herein;    -   A7) U.S. provisional Application No. 60/170,074 entitled        “Optical routing/switching based on control of waveguide-ring        resonator coupling”, filed Dec. 9, 1999 in the name of Amnon        Yariv, said provisional application being hereby incorporated by        reference in its entirety as if fully set forth herein;    -   A8) U.S. Pat. No. 6,052,495 entitled “Resonator modulators and        wavelength routing switches” issued Apr. 18, 2000 in the names        of Brent E. Little, James S. Foresi, and Hermann A. Haus, said        patent being hereby incorporated by reference in its entirety as        if fully set forth herein;    -   A9) U.S. Pat. No. 6,101,300 entitled “High efficiency channel        drop filter with absorption induced on/off switching and        modulation” issued Aug. 8, 2000 in the names of Shanhui Fan,        Pierre R. Villeneuve, John D. Joannopoulos, Brent E. Little, and        Hermann A. Haus, said patent being hereby incorporated by        reference in its entirety as if fully set forth herein;    -   A10) U.S. Pat. No. 5,926,496 entitled “Semiconductor        micro-resonator device” issued Jul. 20, 1999 in the names of        Seng-Tiong Ho and Deanna Rafizadeh, said patent being hereby        incorporated by reference in its entirety as if fully set forth        herein; and    -   A11) U.S. Pat. No. 6,009,115 entitled “Semiconductor        micro-resonator device” issued Dec. 28, 1999 in the name of        Seng-Tiong Ho, said patent being hereby incorporated by        reference in its entirety as if fully set forth herein.    -   A12) U.S. provisional Application No. 60/257,218 entitled        “Waveguides and resonators for integrated optical devices and        methods of fabrication and use thereof”, filed Dec. 21, 2000 in        the name of Oskar J. Painter, said provisional application being        hereby incorporated by reference in its entirety as if fully set        forth herein;    -   A13) U.S. provisional Application No. 60/257,248 entitled        “Modulators for resonant optical power control devices and        methods of fabrication and use thereof”, filed Dec. 21, 2000 in        the names of Oskar J. Painter, Kerry J. Vahala, Peter C. Sercel,        and Guido Hunziker, said provisional application being hereby        incorporated by reference in its entirety as if fully set forth        herein;    -   A14) U.S. provisional Application No. 60/301,519 entitled        “Waveguide-fiber Mach-Zender interferometer and methods of        fabrication and use thereof”, filed Jun. 27, 2001 in the names        of Oskar J. Painter, David W. Vernooy, and Kerry J. Vahala, said        provisional application being hereby incorporated by reference        in its entirety as if fully set forth herein;    -   A15) U.S. non-provisional application Ser. No. 09/788,303        entitled “Cylindrical processing of optical media”, filed Feb.        16, 2001 in the names of Peter C. Sercel, Kerry J. Vahala,        David W. Vernooy, and Guido Hunziker, said non-provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein.    -   A16) U.S. non-provisional application Ser. No. 09/788,331        entitled “Fiber-ring optical resonators”, filed Feb. 16, 2001 in        the names of Peter C. Sercel, Kerry J. Vahala, David W. Vernooy,        Guido Hunziker, and Robert B. Lee, said non-provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein.    -   A17) U.S. non-provisional application Ser. No. 09/788,300        entitled “Resonant optical filters”, filed Feb. 16, 2001 in the        names of Kerry J. Vahala, Peter C. Sercel, David W. Vernooy,        Oskar J. Painter, and Guido Hunziker, said non-provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein.    -   A18) U.S. non-provisional application Ser. No. 09/788,301        entitled “Resonant optical power control device assemblies”,        filed Feb. 16, 2001 in the names of Peter C. Sercel, Kerry J.        Vahala, David W. Vernooy, Guido Hunziker, Robert B. Lee, and        Oskar J. Painter, said non-provisional application being hereby        incorporated by reference in its entirety as if fully set forth        herein.    -   A19) U.S. provisional Application No. 60/335,656 entitled        “Polarization-engineered transverse-optical-coupling apparatus        and methods”, Docket No CQC12P, filed Oct. 30, 2001 in the names        of Kerry J. Vahala, Peter C. Sercel, Oskar J. Painter, David W.        Vernooy, and David S. Alavi, said provisional application being        hereby incorporated by reference in its entirety as if fully set        forth herein;    -   A20) U.S. provisional Application No. 60/334,705 entitled        “Integrated end-coupled transverse-optical-coupling apparatus        and methods”, Docket No. CQC15P, filed Oct. 30, 2001 in the        names of Henry A. Blauvelt, Kerry J. Vahala, Peter C. Sercel,        Oskar J. Painter, and Guido Hunziker, said provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein;    -   A21) U.S. provisional Application No. 60/333,236 entitled        “Alignment apparatus and methods for transverse optical        coupling”, Docket No. CQC16P, filed Nov. 23, 2001 in the names        of Charles I. Grosjean, Guido Hunziker, Paul M. Bridger, and        Oskar J. Painter, said provisional application being hereby        incorporated by reference in its entirety as if fully set forth        herein;    -   A22) U.S. non-provisional application Ser. No. 10/037,146        entitled “Resonant optical modulators”, filed Dec. 21, 2001 in        the names of Oskar J. Painter, Peter C. Sercel, Kerry J. Vahala,        David W. Vernooy, and Guido Hunziker, said non-provisional        application being hereby incorporated by reference in its        entirety as if fully set forth herein.

This application is also related to subject matter disclosed in thefollowing publications, each of said publications being herebyincorporated by reference in its entirety as if fully set forth herein:

-   P1) Ming Cai, Guido Hunziker, and Kerry Vahala, “Fiber-optic    add-drop device based on a silica microsphere whispering gallery    mode system”, IEEE Photonics Technology Letters Vol. 11 686 (1999);-   P2) J. C. Knight, G. Cheung, F. Jacques, and T. A. Birks,    “Phased-matched excitation of whispering gallery-mode resonances by    a fiber taper”, Optics Letters Vol. 22 1129 (1997);-   P3) R. D. Pechstedt, P. St. J. Russell, T. A. Birks, and F. D.    Lloyd-Lucas, “Selective coupling of fiber modes with use of    surface-guided Bloch modes supported by dieletric multilayer    stacks”, J. Opt. Soc. Am. A Vol. 12(12) 2655 (1995);-   P4) R. D. Pechstedt, P. St. J. Russell, “Narrow-band in-line fiber    filter using surface-guided Bloch modes supported by dielectric    multilayer stacks”, J. Lightwave Tech. Vol. 14(6) 1541 (1996);-   P5) Hiroshi Wada, Takeshi Kamijoh, and Yoh Ogawa, “Direct bonding of    InP to different materials for optical devices”, Proceedings of the    third international symposium on semiconductor wafer bonding:    Physics and applications, Electrochemical Society Proceedings,    Princeton N.J., Vol. 95-7, 579-591 (1995);-   P6) R. H. Horng, D. S. Wuu, S. C. Wei, M. F. Huang, K. H.    Chang, P. H. Liu, and K. C. Lin, “AlGaInP/AuBe/glass light emitting    diodes fabricated by wafer-bonding technology”, Appl. Phys. Letts.    Vol. 75(2) 154 (1999);-   P7) Y. Shi, C. Zheng, H. Zhang, J. H. Bechtel, L. R. Dalton, B. B.    Robinson, W. Steier, “Low (sub-1-volt) halfwave voltage polymeric    electro-optic modulators achieved by controlling chromophore shape”,    Science Vol. 288, 119 (2000);-   P8) E. L. Wooten, K. M. Kissa, and A. Yi-Yan, “A review of lithium    niobate modulators for fiber-optic communications systems”, IEEE J.    Selected Topics in Quantum Electronics, Vol. 6(1), 69 (2000);-   P9) D. L. Huffaker, H. Deng, Q. Deng, and D. G. Deppe, “Ring and    stripe oxide-confined vertical-cavity surface-emitting lasers”,    Appl. Phys. Lett., Vol. 69(23), 3477 (1996);-   P10) Serpenguzel, S. Arnold, and G. Griffel, “Excitation of    resonances of microspheres on an optical fiber”, Opt. Lett. Vol. 20,    654 (1995);-   P11) F. Treussart, N. Dubreil, J. C. Knight, V. Sandoghar, J.    Hare, V. Lefevre-Seguin, J. M. Raimond, and S. Haroche, “Microlasers    based on silica microspheres”, Ann. Telecommun. Vol. 52, 557 (1997);-   P12) M. L. Gorodetsky, A. A. Savchenkov, V. S. Ilchenko, “Ultimate Q    of optical microsphere resonators”, Optics Letters, Vol. 21, 453    (1996);-   P13) Carl Arft, Diego R. Yankelovich, Andre Knoesen, Erji Mao, and    James S. Harris Jr., “In-line fiber evanescent field electrooptic    modulators”, Journal of Nonlinear Optical Physics and Materials Vol.    9(1) 79 (2000);-   P14) Pochi Yeh, Amnon Yariv, and Chi-Shain Hong, “Electromagnetic    propagation in periodic stratified media. I. General theory”, J.    Optical Soc. Am., Vol. 67(4) 423 (1977);-   P15) Ming Cai, Oskar Painter, and Kerry J. Vahala, “Observation of    critical coupling in a fiber taper to a silica-microsphere    whispering-gallery mode system”, Physical Review Letters, Vol. 85(1)    74 (2000);-   P16) M. Kondow, T. Kitatani, S. Nakatsuka, M. C. Larson, K.    Nakahara, Y. Yazawa, M. Okai, and K. Uomi, “GaInNAs: A novel    material for long-wavelength semiconductor lasers”, IEEE Journal of    Selected Topics in Quantum Electronics, Vol 3(3), 719 (1997);-   P17) H. Saito, T. Makimoto, and N. Kobayashi, “MOVPE growth of    strained InGaAsN/GaAs quantum wells”, J. Crystal Growth, Vol. 195    416 (1998);-   P18) W. G. Bi and C. W. Tu, “Bowing parameter of the band-gap energy    of GaN_(x)As_(1-x)”, Appl. Phys. Lett. Vol. 70(12) 1608 (1997);-   P19) H. P. Xin and C. W. Tu, “GaInNAs/GaAs multiple quantum wells    grown by gas-source molecular beam epitaxy”, Appl. Phys Lett. Vol.    72(19) 2442 (1998);-   P20) B. Koley, F. G. Johnson, O. King, S. S. Saini, and M. Dagenais,    “A method of highly efficient hydrolization oxidation of III-V    semiconductor lattice matched to indium phosphide”, Appl. Phys.    Lett. Vol. 75(9) 1264 (1999);-   P21) Z. J. Wang, S.-J. Chua, F. Zhou, W. Wang, and R. H. Wu, “Buried    heterostructures InGaAsP/InP strain-compensated multiple quantum    well laser with a native-oxidized InAlAs current blocking layer”,    Appl. Phys. Lett. Vol 73(26) 3803 (1998);-   P22) N. Ohnoki, F. Koyama, and K. Iga, “Superlattice    AlAs/AlInAs-oxide current aperture for long wavelength InP-based    vertical-cavity surface-emitting laser structure”, Appl. Phys. Lett.    Vol. 73(22) 3262 (1998);-   P23) N. Ohnoki, F. Koyama, and K. Iga, “Super-lattice AlAs/AlInAs    for lateral-oxide current confinement in InP-based lasers”, J.    Crystal Growth Vol. 195 603 (1998);-   P24) K. D. Choquette, K. M. Geib, C. I. H. Ashby, R. D. Twesten, O.    Blum, H. Q. Hou, D. M. Follstaedt, B. E. Hammons, D. Mathes, and R.    Hull, “Advances in selective wet oxidation of AlGaAs alloys”, IEEE    Journal of Selected Topics in Quantum Electronics Vol. 3(3) 916    (1997);-   P25) M. H. MacDougal, P. D. Dapkus, “Wavelength shift of selectively    oxidized Al_(x)O_(y)—AlGaAs—GaAs distributed Bragg reflectors”, IEEE    Photonics Tech. Lett. Vol. 9(7) 884 (1997);-   P26) C. I. H. Ashby, M. M. Bridges, A. A. Allerman, B. E. Hammons,    “Origin of the time dependence of wet oxidation of AlGaAs”, Appl.    Phys. Lett. Vol. 75(1) 73 (1999);-   P27) P. Chavarkar, L. Zhao, S. Keller, A. Fisher, C. Zheng, J. S.    Speck, and U. K. Mishra, “Strain relaxation of In_(x)Ga_(1-x)As    during lateral oxidation of underlying AlAs layers”, Appl. Phys.    Lett. Vol. 75(15) 2253 (1999);-   P28) R. L. Naone and L. A. Coldren, “Surface energy model for the    thickness dependence of the lateral oxidation of AlAs”, J. Appl.    Phys. Vol. 82(5) 2277 (1997);-   P29) M. H. MacDougalP. D. Dapkus, A. E. Bond, C.-K. Lin, and J.    Geske, “Design and fabrication of VCSEL's with Al_(x)O_(y)-GaAs    DBR's”, IEEE Journal of Selected Topics in Quantum Electronics Vol.    3(3) 905 (1997);-   P30) E. I. Chen, N. Holonyak, Jr., and M. J. Ries, “Planar disorder-    and native-oxide-defined photopumped AlAs-GaAs superlattice minidisk    lasers”, J. Appl. Phys. Vol. 79(11) 8204 (1996); and    -   P31) Y. Luo, D. C. Hall, L. Kou, L. Steingart, J. H. Jackson, O.        Blum, and H. Hou, “Oxidized Al_(x)Ga_(1-x)As heterostructures        planar waveguides”, Appl. Phys. Lett. Vol. 75(20) 3078 (1999);-   P32) B. Pezeshki, J. A. Kash, and F. Agahi, “Waveguide version of an    asymmetric Fabry-Perot modulator”, Appl. Phys. Lett. Vol. 67(12)    1662 (1995);-   P33) B. Pezeshki, F. F. Tong, J. A. Kash, and D. W. Kisker,    “Vertical cavity devices as wavelength selective waveguides”, J.    Lightwave Tech. Vol. 12(10) 1791 (1994);-   P34) F. Agahi, B. Pezeshki, J. A. Kash, and D. W. Kisker,    “Asymmetric Fabry-Perot modulator with a waveguide geometry”,    Electron. Lett. Vol. 32(3) 210 (1996);-   P35) B. Pezeshki, J. A. Kash, D. W. Kisker, and F. Tong, “Multiple    wavelength light source using an asymmetric waveguide coupler”,    Appl. Phys. Lett. Vol. 65(2) 138 (1994); and-   P36) B. Pezeshki, J. A. Kash, D. W. Walker, and F. Tong, “Wavelength    sensitive tapered coupler with anti-resonant waveguides”, IEEE Phot.    Tech. Lett. Vol. 6(10) 1225 (1994).

Optical fiber and propagation of high-speed optical pulses therethroughhas become the technology of choice for high-speed telecommunications.Generation of trains of such high-speed optical pulses, representativeof voice, video, data, and/or other signals, requires high-speed opticalmodulation techniques, typically intensity modulation techniques. Directintensity modulation of the light source (usually a laser diode)generally induces unwanted phase and/or frequency modulation as well,which may be problematic when the modulated optical mode must propagatelong distances within the optical fiber, or when the modulated opticalmode is one narrow-linewidth wavelength component among many within awavelength division multiplexed (WDM) fiber-optic telecommunicationsystem. It has therefore become standard practice to provide an externalintensity modulator as a separate optical component, to act on apropagating optical mode after it has left the light source.

Other devices may be required for subsequent manipulation and/or controlof the propagating optical pulse train, including but not limited to,for example, routers, switches, fixed and variable attenuators, fixedand variable couplers, bi-directional couplers, channel add-dropfilters, N×N switches, and so forth. It is desirable for these devicesto perform their respective functions without the need for conversion ofthe optical pulse train into an electronic signal for manipulation andre-conversion to an optical pulse train following manipulation. It ispreferable for these devices to perform their respective functions bydirect manipulation of the optical pulse train. To this end many ofthese devices are fabricated as integrated devices, with opticalportions and electronically-driven control portions fabricated as asingle integrated component. Many of these devices function bycontrolling flow of optical power from one optical mode to anotheroptical mode in a controlled fashion. For example, optical power may beshifted from a propagating optical mode of a first optical fiber to apropagating mode of a second optical fiber in an actively-controlledfashion, using so-called directional couplers, or in awavelength-dependent fashion (active or passive), using so-calledchannel add-drop filters. Application of a control signal to an activedevice may cause optical power to remain within a propagating opticalmode of a first optical fiber, or to couple into a propagating opticalmode of a second optical fiber.

High insertion losses associated with currently available devicesnecessitate use of optical amplifiers to boost optical signal levels ina fiber-optic telecommunications system, essentially to replace opticalpower thrown away by the use of lossy modulators, couplers, and otherdevices. This adds significantly to the cost, size, and powerconsumption of any fiber-optic system or sub-system. Furthermore, thefull potential of powerful new on-chip integrated optical devices cannotbe realized when a substantial fraction of the optical signal is lostthrough inefficient transfer of optical power between an optical fiberand a waveguide on an integrated optical chip.

Optical signal power transfer between various optical devices in afiber-optic telecommunications system relies on optical coupling betweenoptical modes in the devices. Transverse-coupling (also referred to astransverse optical coupling, evanescent coupling, evanescent opticalcoupling, directional coupling, directional optical coupling) may beemployed, thereby eliminating transverse mode matching requirementsimposed by end-coupling. Such optical power transfer bytransverse-coupling depends in part on the relative modal indices of thetransverse-coupled optical modes. Active control of the modal index ofone or both of the transverse-coupled optical modes would thereforeenable active control of the degree to which optical power istransferred from one device to the other, preferably using controlvoltages substantially smaller magnitude than required by currentlyavailable devices. Optical power transfer from a fiber-optic or otherlow-index optical waveguide to an integrated on-chip optical device(typically higher-index) could be greatly improved by employingtransverse-coupling. Such optical power transfer could be activelycontrolled by controlling a modal index of an optical mode of awaveguide and/or resonator of the integrated device. Optical losseswithin such an integrated on-chip device could also be reduced.

FIG. 1 shows an example of a modulator 10 fabricated as an opticalwaveguide Mach-Zender interferometer on an electro-optic crystalsubstrate 12 (typically lithium niobate). Standard fabricationtechniques are used to fabricate the waveguide 14 (usually lithographicmasking followed by ion diffusion) and to deposit control electrodes 16.An incident optical signal propagating into entrance face 18 (i.e.,“end-coupled”) and through the device is divided into the two arms ofthe interferometer waveguide 14, application of a control voltage acrossthe control electrodes 16 (in any of several configurations) induces arelative change in the modal indices of the optical modes in the arms(by an electro-optic mechanism), and the optical signals propagating inthe arms are then recombined before exiting through exit face 19.Variation of the control voltage enables modulation of the transmissionof the incident optical signal from a lower operational opticaltransmission level (when the recombined optical modes substantiallydestructively interfere; preferably near zero transmission) to an upperoperational optical transmission level (when the recombined opticalmodes substantially constructively interfere; preferably near 100%transmission, but typically limited by insertion loss of the modulator).Modulators of this sort are widely used in fiber optictelecommunications systems, may enable modulation frequencies up toseveral tens of GHz, and may require control voltages of at leastseveral volts up to about 10 volts for substantially full modulation ofthe optical signal. The control voltage required for a device to achievesubstantially full modulation (i.e., near zero transmission of theoptical mode at the lower transmission level) is referred to as V_(π),since a phase shift of about π is required to make the optical modespropagating in the two arms of the interferometer substantiallydestructively interfere. V₉₀ is an important figure-of-merit forcharacterizing electro-optic modulators. The relatively high V_(π) oftypical lithium niobate modulators forces the use of expensive highspeed electronic drivers (described below), increasing cost and powerconsumption of the device. In addition, coupling optical power into andout of the faces of the modulator (end-fire coupling, or end-coupling)is quite inefficient, and typical lithium niobate modulators may haveinsertion losses as high as 6 dB. Most of the insertion loss may beattributed to transverse mode mismatch of the input optical mode and thepreferred mode of the waveguide. This may be somewhat mitigated bymodifying the device to achieve better mode-matching, but at the expenseof a larger V_(π).

FIG. 2 shows an example of a directional coupler 20 (also referred to asa 2×2 optical switch) fabricated as an integrated optical device on anelectro-optic crystal substrate 22 (typically lithium niobate). In thisexample two waveguides 24 a and 24 b are fabricated on the substrate,and are positioned in relatively close proximity in a coupling portionof the device. In this way an optical signal propagating in an opticalmode of one waveguide may transverse-couple into an optical mode of thesecond waveguide. The device is typically constructed so that over thelength of the coupling portion, substantially all of the optical powerentering the first waveguide is transferred into the second waveguide.Control electrodes 26 are positioned so that an applied control voltagealters the relative modal indices of the optical modes of the twowaveguides in the coupling portion (by an electro-optic mechanism). Aswitching voltage V₀, typically several volts up to about 10 volts, isthe voltage that alters the relative modal indices (i.e., the phasematching condition between the waveguides) to the extent thatsubstantially none of the optical power entering the first waveguide istransferred to the second waveguide. By switching the control voltagebetween about zero volts and about V₀, the optical power entering thefirst waveguide may be switched between exiting via the second waveguide(zero volts applied) or exiting via the first waveguide (V₀ applied).Such couplers may exhibit switching frequencies of up to 10 GHz, and V₀is an important figure-of-merit for characterizing electro-opticcouplers. In a manner similar to the modulators described hereinabove,these devices require costly high-speed electronic driver hardware(described below) and exhibit insertion losses as high as 6 dB.

A general discussion of electro-optic modulators, interferometers, andcouplers may be found in Fundamentals of Photonics by B. E. A. Saleh andM. C. Teich (Wiley, New York, 1991), hereby incorporated by reference inits entirety as if fully set forth herein. Particular attention iscalled to Chapter 18.

For operating voltages (V_(π) or V₀) on the order of several volts andhigh modulation/switching frequencies, a high speed electronic controlinput signal must typically be amplified to the appropriate level forapplication to the device by a high speed electronic amplifier, usuallyreferred to as a driver or RF driver. A driver adds substantially to thesize, cost, and power consumption of current optical modulators,couplers, and other devices, and may limit the maximum frequency atwhich such devices may be driven. For operating voltages (V_(π) or V₀)on the order of 10 V, a device may consume on the order of 1 W ofelectrical drive power. This power must be dissipated and/or otherwisemanaged properly to avoid overheating, degraded performance, and/oreventual failure of the device. This may be particularly problematicwhen the properties defining the performance of the device (such aswaveguide pathlength, refractive and modal indices, and so forth) aretemperature dependent. Since such large numbers of such modulators,couplers, switches, and other optical devices are required to implementa fiber-optic telecommunications system of any significant extent(organization-, city-, state-, nation-, and/or world-wide; alternativelyenterprise, metro, and/or trunk systems), any potential reductions insize, cost, and/or power consumption may prove to be quite significant.A sub-volt control voltage level (V_(π) and/or V₀) would eliminate theneed for a driver, potentially cutting the cost of each device, andwould result in a corresponding decrease in power consumption and itsattendant technical difficulties and economic disadvantages. Limitationson operating speed imposed by driver performance would be eliminated.

It is desirable to provide optical modulators, interferometers,couplers, routers, add/drop filters, switches, and/or other deviceswherein optical power may be efficiently transferred to/from the devicefrom/to an optical fiber or other low-index waveguide withoutlimitations and/or insertion losses imposed by end-coupling. It isdesirable to provide optical modulators, interferometers, couplers,routers, add/drop filters, switches, and/or other devices whereinoptical power may be efficiently transferred to/from the device from/toan optical fiber or other low-index waveguide by transverse-coupling. Itis desirable to provide optical modulators, interferometers, couplers,routers, add/drop filters, switches, and/or other devices havinginsertion loss less than about 3 dB. It is desirable to provide opticalmodulators, interferometers, couplers, routers, add-drop filters,switches, and/or other integrated optical devices that may be wellmodal-index-matched to optical fiber and/or other low-index waveguides.It is desirable to provide optical modulators, interferometers,couplers, routers, add-drop filters, and/or other devices that may befabricated as integrated optical devices, on a planar platform or onmultiple-level vertically-integrated planar platforms. It is thereforedesirable to provide optical modulators, interferometers, couplers,routers, add-drop filters, switches, and/or other devices wherein therequired control voltage level (V_(π) or V₀) is less than about onevolt. It is desirable to provide optical modulators, interferometers,couplers, routers, add-drop filters, and/or other devices that do notrequire a driver for amplifying electronic control signals. It isdesirable to provide optical modulators, interferometers, couplers,routers, add-drop filters, and/or other devices that are compatible withother extant components of a fiber-optic telecommunications system.

SUMMARY

Certain aspects of the present invention may overcome one or moreaforementioned drawbacks of the previous art and/or advance thestate-of-the-art of apparatus and methods for modulating, routing,and/or other optical power control devices, and in addition may meet oneor more of the following objects:

-   -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof compatible        with other extant components of a fiber-optic telecommunications        system;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof that may be        fabricated as integrated optical devices;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof that may        yield optical devices having insertion loss of less than about 3        dB;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        optical power may be efficiently transferred to/from the device        from/to an optical fiber or other low-index waveguide;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        transverse-coupling serves to transfer optical signals to/from        the waveguide/resonator;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof that may        include a laterally-confined optical waveguide segment;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof that may        include a multi-layer optical waveguide segment;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof that may        include a dispersion-engineered waveguide segment;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof that may        include a ridge-like waveguide/resonator structure protruding        from a substrate;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof for enabling        modal-index-matching between the waveguide/resonator and an        optical fiber or other low-index waveguide;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof for enabling        passive modal-index-matching between the waveguide/resonator and        an optical fiber or other low-index waveguide;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof for enabling        active modal-index-matching between the waveguide/resonator and        an optical fiber or other low-index waveguide by application of        a control signal to the waveguide/resonator;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein a        multi-layer stack guides a surface-guided optical mode, the        surface-guided optical mode being transverse-coupled to a mode        of another optical element;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein a        pair of multi-layer stacks guide a substantially confined        optical mode therebetween, the confined optical mode being        transverse-coupled to a mode of another optical element;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        waveguide/resonator includes an at least one electro-active        layer and electronic control components for controlling the        electro-active layer;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        dispersive properties of the multi-layer stack(s) enable        substantial changes in the modal index and/or modal loss of a        guided optical mode by application of relatively small control        voltages to the electro-active layer;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        waveguide/resonator includes at least one non-linear-optical        layer and optical control components for controlling the        non-linear-optical layer;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        dispersive properties of the multi-layer stack(s) enable        substantial changes in the modal index and/or modal loss of a        guided optical mode by application of relatively small optical        control signals to the non-linear-optical layer;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        required control voltage level (V_(π) or V₀) may be less than        about one volt;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein use        of a driver for amplifying high-data-rate electronic control        signals may not be required;    -   To provide fiber-optic modulators and methods of fabrication and        use thereof wherein a simplified driver for amplifying        high-data-rate electronic control signals may be employed;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        waveguide/resonator further comprises lateral lower-index        portions for substantially laterally confining a guided optical        mode;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        lateral lower-index portions of the waveguide/resonator restrict        the guided optical modes to one or a few transverse optical        modes;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        lateral lower-index portions of the waveguide/resonator decrease        optical loss and/or increase the Q-factor of the        waveguide/resonator;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        multi-layer stack(s), electro-active and/or non-linear-optical        layer, and control components (if present) may be fabricated by        a layer growth/deposition sequence on a single substrate;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein the        multi-layer stack(s), electro-active and/or non-linear-optical        layer, and control components (if present) may be fabricated by        layer growth/deposition sequences on multiple substrates        followed by wafer-bonding of the grown/deposited layers;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        lateral low-index portions of the waveguide/resonator may be        provided by lateral chemical conversion of one or more layers of        the waveguide/resonator;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein one        or more grown/deposited layers of the waveguide/resonator may        subsequently be substantially completely converted to another        material through lateral chemical conversion;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        control of a modal index of a guided optical mode enables        control of optical power transfer between the        waveguide/resonator and another optical element through        transverse-coupling;    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        control of a modal index of a guided resonant optical mode        enables control of a resonance frequency of the resonator; and    -   To provide waveguides and resonators for integrated optical        devices and methods of fabrication and use thereof wherein        control of a modal index and/or optical loss of a guided optical        mode enables control of an operational state of the optical        device.

One or more of the foregoing objects may be achieved in the presentinvention by an optical waveguide/resonator including at least onemulti-layer laterally-confined dispersion-engineered optical waveguidesegment. The multi-layer waveguide segment may include a singlemulti-layer reflector stack for guiding a surface-guided optical mode(SGOM). The multi-layer waveguide segment may also include laterallower-index portions thereof for lateral confinement of thesurface-guided optical mode, and may include a waveguide or core layerthereon. The multi-layer waveguide segment may further comprise one ormore electro-active and/or non-linear-optical layers and controlcomponents for controlling the refractive index and/or optical lossthereof. Strongly dispersive optical properties of thesingle-reflector-guided SGOM (a substantially flat dispersion relationin the operating wavelength range, so that a narrow range of wavelengthscover a wide range of propagation constants or modal indices) serve toproduce a substantially larger modal index shift of the SGOM for a givenapplied control signal than previous devices. The surface-guided opticalmode may be transverse-coupled to another optical mode of anotheroptical element. Control of the modal index and/or optical loss mayenable control of: optical power transfer between thewaveguide/resonator and another optical element; the resonance frequencyof a resonant optical mode of a resonator; and an operational state ofthe waveguide/resonator and/or an optical device incorporating thewaveguide/resonator.

Alternatively, the multi-layer waveguide segment may include a pair ofmulti-layer reflector stacks for guiding a substantially confinedoptical mode along a waveguide or core layer therebetween. Such adual-reflector waveguide segment may also include lateral lower-indexportions thereof for lateral confinement of the guided optical mode. Thedual-reflector waveguide segment may further comprise one or moreelectro-active and/or non-linear-optical layers and control componentsfor controlling the refractive index and/or optical loss thereof. Thestrongly dispersive optical properties of the dual-reflector-guidedoptical mode (a substantially flat dispersion relation in the operatingwavelength range, so that a narrow range of wavelengths cover a widerange of propagation constants or modal indices) serve to produce asubstantially larger modal index shift of the guided optical mode for agiven applied control signal than previous devices. The guided opticalmode may be transverse-coupled to another optical mode of anotheroptical element. Control of the modal index and/or optical loss mayenable control of: optical power transfer between thewaveguide/resonator and another optical element; the resonance frequencyof a optical mode of a resonator; and an operational state of thewaveguide/resonator and/or an optical device incorporating thewaveguide/resonator.

Multi-layer waveguides/resonators may be fabricated by 1) “verticalfabrication” of a multi-layer structure including reflector stack(s),any required core or waveguide layer, any required electro-active and/ornon-linear-optical layer(s), any required control components, and anyother desired layers on a substrate, followed by 2) “horizontalfabrication” of the multi-layer structure by spatially-selectiveprocessing of portions of the multi-layer structure, creating on thesubstrate a multi-layer waveguide segment of the desired size, shape,and topology. The “horizontal fabrication” step may further includelateral processing of one or more layers of the multi-layer waveguidesegment, resulting in chemical conversion of all or part of the affectedlayers.

Additional objects and advantages of the present invention may becomeapparent upon referring to the preferred and alternative embodiments ofthe present invention as illustrated in the drawings and described inthe following written description and/or claims.

BRIEF DESCRIPTION OF THE DRAWINGS

In many of the Figures, a reference coordinate system is shown fordescriptive convenience only.

FIG. 1 shows a prior-art Mach-Zender interferometer modulator fabricatedon an electro-optic crystal substrate.

FIG. 2 shows a prior-art directional coupler fabricated on anelectro-optic crystal substrate.

FIGS. 3A, 3B, and 3C are top plan, transverse-sectional, and sideelevation views, respectively, of an optical waveguide on a substrateaccording to the present invention, showing a direction of lightpropagation therein.

FIG. 4 is an isometric view of an optical waveguide on a substrateaccording to the present invention, showing a direction of lightpropagation therein.

FIGS. 5A, 5B, and 5C are top plan, transverse-sectional, and sideelevation views, respectively, of an optical resonator on a substrateaccording to the present invention, showing a direction of lightpropagation therein.

FIG. 6 is an isometric view of an optical resonator on a substrateaccording to the present invention, showing a direction of lightpropagation therein.

FIG. 7 is a transverse-sectional view of a single-DBR waveguide of thepresent invention positioned on a substrate.

FIG. 8 is a transverse-sectional view of a single-DBR waveguide of thepresent invention, having a core layer thereon and positioned on asubstrate.

FIG. 9 is a transverse-sectional view of an asymmetric-dual-DBRwaveguide of the present invention positioned on a substrate.

FIG. 10 is a transverse-sectional view of a dual-DBR waveguide of thepresent invention positioned on a substrate.

FIG. 11 is a transverse-sectional view of a single-DBR waveguide of thepresent invention, having lateral low-index portions thereon andpositioned on a substrate.

FIG. 12 is a transverse-sectional view of a single-DBR waveguide of thepresent invention, having a core layer and lateral low-index portionsthereon and positioned on a substrate.

FIG. 13 is a transverse-sectional view of an asymmetric-dual-DBRwaveguide of the present invention, having lateral low-index portionsthereon and positioned on a substrate.

FIG. 14 is a transverse-sectional view of a dual-DBR waveguide of thepresent invention, having lateral low-index portions thereon andpositioned on a substrate.

FIG. 15 is a flow diagram for a single-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 16 is a process diagram for a single-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 17 is a flow diagram for a single-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 18 is a process diagram for a single-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 19 is a flow diagram for a single-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 20 is a process diagram for a single-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 21 is a flow diagram for a two-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 22 is a process diagram for a two-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 23 is a flow diagram for a two-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 24 is a process diagram for a two-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 25 is a flow diagram for a two-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 26 is a process diagram for a two-substrate vertical fabricationprocedure for a single-DBR waveguide of the present invention.

FIG. 27 is a flow diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 28 is a process diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 29 is a flow diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 30 is a process diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 31 is a flow diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 32 is a flow diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 33 is a flow diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 34 is a process diagram for a single-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 35 is a flow diagram for a three-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 36 is a process diagram for a three-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 37 is a flow diagram for a three-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 38 is a process diagram for a three-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 39 is a flow diagram for a three-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIG. 40 is a process diagram for a three-substrate vertical fabricationprocedure for a dual-DBR waveguide of the present invention.

FIGS. 41A and 41B are process diagrams for horizontal fabrication of awaveguide of the present invention.

FIGS. 42A and 42B are process diagrams for horizontal fabrication of aresonator of the present invention.

FIGS. 43A and 43B are process diagrams for horizontal fabrication of aresonator of the present invention.

FIG. 44 is a process diagram for horizontal fabrication of a waveguideof the present invention.

FIG. 45 is a process diagram for horizontal fabrication of a waveguideof the present invention.

FIG. 46 is a process diagram for horizontal fabrication of a waveguideof the present invention.

FIG. 47 is a process diagram for horizontal fabrication of a waveguideof the present invention.

FIG. 48 is a process diagram for horizontal fabrication of a waveguideof the present invention.

FIG. 49 is a transverse-sectional view of a waveguide of the presentinvention having asymmetric lateral low-index portions thereon.

FIG. 50 is a transverse-sectional view of a waveguide of the presentinvention having asymmetric lateral low-index portions thereon.

FIG. 51 shows a fiber-optic taper transverse-coupled to an opticalwaveguide on a substrate according to the present invention.

FIG. 52 shows a Mach-Zender interferometer optical modulator on asubstrate according to the present invention.

FIG. 53 shows a fiber-optic taper transverse-coupled to a Mach-Zenderinterferometer optical modulator on a substrate according to the presentinvention.

FIG. 54 shows a fiber-optic taper transverse-coupled to a Mach-Zenderinterferometer optical modulator on a substrate according to the presentinvention.

FIG. 55 shows an optical switch on a substrate according to the presentinvention.

FIG. 56 shows a pair of fiber-optic tapers transverse-coupled to anoptical switch on a substrate according to the present invention.

FIG. 57 shows a fiber-optic taper transverse-coupled to an opticalresonator on a substrate in turn transverse-coupled to a loss-controloptical waveguide according to the present invention.

FIGS. 58 and 59 show a fiber-optic taper transverse-coupled to anoptical waveguide on a substrate according to the present invention soas to form a Mach-Zender interferometer optical modulator.

FIGS. 60 and 61 show a fiber-optic taper transverse-coupled to anoptical waveguide on a substrate according to the present invention soas to form a Mach-Zender interferometer optical modulator.

FIGS. 62A and 62B show examples of multi-layer and/or periodicstructures employed for lateral confinement of a guided optical mode ina waveguide.

It should be noted that the relative proportions of various structuresshown in the Figures may be distorted to more clearly illustrate thepresent invention. In particular, various metal, semiconductor, and/orother thin films, layers, and/or coatings may also be shown havingdisproportionate and/or exaggerated thicknesses for clarity. Relativedimensions of various waveguides, resonators, optical fibers/tapers, andso forth may also be distorted, both relative to each other as well astransverse/longitudinal proportions. The text and incorporatedreferences should be relied on for the appropriate dimensions ofstructures shown herein.

DETAILED DESCRIPTION OF PREFERRED AND ALTERNATIVE EMBODIMENTS

For purposes of the present written description, the term “waveguide”shall often be intended encompass both open waveguides (in which noclosed optical path is provided for allowing re-circulation of opticalpower within an optical mode supported by the waveguide) and closedwaveguides (in which a closed optical path is provided for allowingre-circulation of optical power within an optical mode supported by thewaveguide; such closed waveguides may also be equivalently referred toas resonators or rings). The term “waveguide” shall often be used hereinto denote both open and closed structures when structure and/orfabrication of such open and closed waveguides is discussed. In portionsof the written description wherein only one or the other type ofwaveguide (open or closed) is described, it will be made clear in thetext which is intended, either implicitly or explicitly. This willtypically be the case when functional aspects of devices incorporatingthe open and/or closed waveguides are discussed.

For purposes of the present written description and/or claims, the term“laterally-confined waveguide” shall typically denote an opticalstructure elongated along an optical propagation direction (thelongitudinal direction) and adapted so as to substantially confine oneor more optical modes in directions substantially perpendicular to theoptical propagation direction (i.e., transverse directions). Thelongitudinal dimension/direction shall be associated with the terms“forward”, “backward”, and so on. Such laterally-confined waveguides arefrequently fabricated on, mounted on, or otherwise positioned on asubstantially planar portion of a substrate, in which case longitudinaldimensions/directions may also be referred to as “horizontal”.Transverse dimensions/directions may be associated with terms “vertical”and “horizontal” relative to a substrate. Vertical dimensions/directions(hence also transverse relative to the waveguide) may be associated withthe terms “up”, “down”, “above”, “below”, “superior”, “inferior”, “top”,“bottom”, and so on. Horizontal dimensions/directions that are alsotransverse relative to the waveguide may be associated with the terms“left”, “right”, “lateral”, “medial”, “side”, and so on. Suchdescriptive terms are typically intended to convey local directionsand/or positions relative to a waveguide, a substrate, an alignmentstructure, and/or the like, and are typically not intended to conveyabsolute position or direction in space.

An example configuration of an open waveguide 30 on a substrate 32 isshown in FIGS. 3A, 3B, 3C, and 4, while an example configuration of aresonator 50 on substrate 52 is shown in FIGS. 5A, 5B, 5C, and 6. Asshown in these Figures, a reference axis system may be defined relativeto a waveguide or resonator structure on a substrate for convenience ofdescription only, and shall not be construed as limiting the scope ofinventive concepts disclosed and/or claimed herein. The +z-axis shall bedefined generally as the direction of propagation of light along awaveguide or resonator structure (indicated by the larger open arrows),and will typically be oriented substantially parallel to a plane definedby the substrate surface on which the waveguide or resonator ispositioned. It should be noted that such waveguide or resonatorstructures may typically support propagation of light in eitherdirection (i.e., +z or −z). The +y-axis shall be oriented substantiallyperpendicular to and away from the substrate surface plane. The x-axisshall be oriented substantially parallel to the substrate plane andsubstantially perpendicular to the direction of propagation of lightalong the waveguide or resonator. The reference axis system is definedlocally with respect to the waveguide or resonator, so that in the caseof a curved waveguide or resonator, the axis system may vary in itsabsolute orientation in space at various points along the waveguide orresonator, but its orientation with respect to the waveguide at anygiven point is substantially as described hereinabove (note FIGS. 5A,5B, 5C, and 6). The z-direction may be referred to as the longitudinaldirection. The x-direction may be referred to as transverse, horizontal,left, right, lateral, and/or medial, while the y-direction may bereferred to as transverse, vertical, up, down, superior, and/orinferior.

For purposes of the written description and/or claims, “index” maydenote the bulk refractive index of a particular material (also referredto herein as a “material index”) or may denote the propagation constant(in the z-direction) of a particular optical mode in a particularoptical element (referred to herein as a “modal index”). As referred toherein, the term “low-index” shall denote any materials and/or opticalstructures having an index less than about 2.5, while “high-index” shalldenote any materials and/or structures having an index greater thanabout 2.5. Within these bounds, “low-index” may preferably refer tosilicas, glasses, oxides, polymers, and any other optical materialshaving indices between about 1.3 and about 1.8, and may include opticalfiber, optical waveguides, planar lightwave circuit components, and anyother optical components incorporating such materials. Similarly,“high-index” may preferably refer to materials such as semiconductors orany other material having indices of about 3 or greater. The terms“high-index” and “low-index” are to be distinguished from the terms“lower-index” and “higher-index”, also employed herein. “Low-index” and“high-index” refer to an absolute numerical value of the index (greaterthan or less than about 2.5), while “lower-index” and “higher-index” arerelative terms indicating which of two materials has the larger index,regardless of the absolute numerical values of the indices.

For purposes of the written description and/or claims, the term“multi-layer reflector stack” or “MLR stack” or “MLR” shall denote amulti-layer structure wherein the layer index varies with eachsuccessive layer of the stack (typically alternately increasing anddecreasing; often alternating layers of a higher-index material and alower-index material), yielding an optical structure havingwavelength-dependent optical properties. A common example of such astructure is a distributed Bragg reflector (DBR), which may typicallycomprise alternating quarter-wave-thickness layers of a higher-indexmaterial and a lower-index material. Graded-index material(s) may alsobe employed. The term “multi-layer reflector stack” shall denote anyperiodic, partially periodic, multi-periodic, quasi-periodic, and/orsimilar multi-layer varying-index structure.

For purposes of the written description and/or claims, the term“electro-active” shall denote any material that may exhibitelectro-optic and/or electro-absorptive properties. The term“non-linear-optical” shall denote any material that may exhibitnon-linear optical properties, including both resonant and non-resonantnon-linear-optical properties.

For purposes of the written description and/or claims,“transverse-coupling” (also referred to as transverse optical coupling,evanescent coupling, evanescent optical coupling, directional coupling,directional optical coupling) shall generally denote those situations inwhich two optical elements, each capable of supporting a propagatingand/or resonant optical mode and at least one having an evanescentportion of its optical mode extending beyond the respective opticalelement, are optically coupled by at least partial transverse spatialoverlap of the evanescent portion of one optical mode with at least aportion of the other optical mode. The amount, strength, level, ordegree of optical power transfer from one optical element to the otherthrough such transverse optical coupling depends on the spatial extentof the overlap (both transverse and longitudinal), the spectralproperties of the respective optical modes, and the relative spatialphase matching of the respective optical modes (also referred to asmodal index matching). To transfer optical power most efficiently, therespective modal indices of the optical modes (equivalently, therespective modal propagation constants), each in its respective opticalelement, must be substantially equal. Mismatch between these modalindices decreases the amount of optical power transferred by transversecoupling between the optical elements, since the coupled modes getfurther out of phase with each other as each propagates within itsrespective optical element and the direction of the optical powertransfer eventually reverses itself. The propagation distance over whichthe modes interact (i.e., the effective interaction length) and thedegree of modal-index matching (or mis-matching) together influence theoverall flow of optical power between the coupled modes. Optical powertransfer between the coupled modes oscillates with a characteristicamplitude and spatial period as the modes propagate, each in itsrespective optical element.

Neglecting the effects of optical loss in the optical elements, an idealsystem consisting of two coupled modes can be characterized by thefollowing coupled system of equations:$\frac{\partial E_{1}}{\partial z} = {{i\quad\beta_{1}E_{1}} + {i\quad\kappa\quad E_{2}}}$$\frac{\partial E_{2}}{\partial z} = {{i\quad\beta_{2}E_{2}} + {i\quad\kappa^{*}\quad E_{1}}}$where the following definitions apply:

-   -   E_(1,2) amplitudes of the coupled fields;    -   β_(1,2) propagation constants of the coupled fields;    -   κ coupling amplitude resulting from spatial overlap of the        fields;    -   z propagation distance coordinate.        For the purpose of illustration, it is assumed that the coupling        amplitude κ is constant over an interaction distance L. Then, an        incident field of amplitude E₁ that is spatially confined to the        first optical element before interaction will couple to the        other wave guide with a resultant field amplitude E₂(L) at z=L        (where we define z=0 as the start of the coupling region) given        by the following expression,        $\frac{{{E_{2}(L)}}^{2}}{{{E_{1}(0)}}^{2}} = {\frac{{\kappa }^{2}}{q^{2}}{\sin^{2}\left( {q\quad L} \right)}}$        $q^{2} = {{\kappa }^{2} + {\frac{1}{4}{{\Delta\beta}^{2}.}}}$        Consider the modal-index mismatch term (Δβ=β₂−β₁) and the        interaction length in this expression. As is well known, a        condition of modal-index mismatch between the two spatial modes        causes an oscillatory power transfer to occur between the        waveguides as the interaction length is varied. The spatial        period of this oscillation, a so-called “beat length”, can be        defined as the distance over which power cycles back and forth        between the guides. Greater amounts of modal-index mismatch will        reduce the beat length. Also note that the absolute magnitude of        power transfer will diminish with increasing modal-index        mismatch. Finally, it is apparent that increased amounts of        interaction length and/or increased modal-index mismatch will        introduce an increased spectral selectivity to the optical power        transfer.

By controlling the modal-index mismatch and/or transverse spatialoverlap between optical modes, these characteristics may be exploitedfor controlling optical power transfer between optical elements. Forexample, by altering the modal-index mismatch, a device may be switchedfrom a first condition, in which a certain fraction of optical power istransferred from a first optical mode in a first optical element to asecond optical mode in a second optical element (modal-index mismatchset so that the effective interaction length is about half of thecharacteristic spatial period described above), to a second condition inwhich little or no optical power is transferred (modal-index mismatchset so that the effective interaction length is about equal to thecharacteristic spatial period). Further discussion of optical couplingmay be found in Fundamentals of Photonics by B. E. A. Saleh and M. C.Teich (Wiley, New York, 1991), hereby incorporated by reference in itsentirety as if fully set forth herein. Particular attention is called toChapters 7 and 18.

It should be noted that optical waveguides and resonators as describedherein, optical modulators, interferometers, couplers, routers, add-dropfilters, switches, and other devices incorporating such waveguidesand/or resonators, their fabrication, and their use according to thepresent invention are intended primarily for handling optical modeshaving wavelengths between about 0.8 μm and about 1.0 μm (the wavelengthrange typically utilized for so-called short-haul fiber-optictelecommunications) and optical modes having wavelengths between about1.2 μm and about 1.7 μm (the wavelength range typically utilized forso-called long-haul fiber-optic telecommunications). However, thesedevices, methods of fabrication, and methods of use may be adapted foruse at any desired wavelength while remaining within the scope ofinventive concepts disclosed and/or claimed herein.

Optical waveguides and/or resonators according to the present inventionmay typically fall into one of two general categories, or may fall intoan intermediate category. In the first category, illustratedschematically in transverse-section in FIG. 7, the waveguide structure700 comprises a single multi-layer reflector 702 (equivalently, a MLR orMLR stack), shown in the form of an elongated ridge-like structureprotruding from a substrate 710. This category may also include awaveguide structure 800 as shown schematically in transverse section inFIG. 8, comprising a MLR stack 802 and a core or waveguide layer 804 onsubstrate 810. Such a single-MLR waveguide may support propagation of anoptical mode as a surface-guided optical mode (equivalently, a SGOM, SGOmode, SG mode, surface-guided mode, optical SGM, SGM, or SG opticalmode; such modes have been referred to in the literature assurface-guided Bloch modes (SGBM), anti-resonant reflecting opticalwaveguide modes (ARROW), and so forth). Such an optical mode is confinedand guided from below by the reflectivity of the MLR stack 702 (802),and from above by the index contrast between the MLR stack 702 (corelayer 804) and a surrounding lower-index medium (air; vacuum; alower-index glass, polymer, semi-conductor, electro-optic, or otherover-layer). Lateral confinement of the SGOM may arise from a similarindex contrast between the sides of an elongated, ridge-like MLR stack702 (MLR stack 802 and core layer 804) and a surrounding lower-indexmedium. Alternatively, some or all of the MLR layers may be providedwith uni-lateral and/or bilateral lower-index portions for laterallyconfining the SGOM within the MLR stack, as shown schematically intransverse-section in FIGS. 11 and 12. FIG. 11 shows a waveguide 1100 onsubstrate 1110, waveguide 1100 comprising a single MLR stack 1102 andlateral lower-index portions 1103. FIG. 12 shows a waveguide 1200 onsubstrate 1210, waveguide 1200 comprising a single MLR stack 1202,lateral lower-index portions thereof 1203, core layer 1204, and laterallower-index portions thereof 1205. In FIG. 12, one or the other or bothof MLR stack 1202 and core layer 1204 may be provided with respectivelower-index lateral portions 1203 and 1205. Lateral confinement mayalternatively be provided by lateral metallic coatings, lateraldielectric coatings, lateral multi-layer reflectors or distributed Braggreflectors, and/or internal reflection at a waveguide lateral surface.In any of FIGS. 7, 8, 11, or 12, evanescent portions of the SG opticalmode may extend upward from the top of the MLR stack and/or laterallyfrom one or both sides of the MLR stack, thereby enablingtransverse-coupling between the SG optical mode and other optical modessufficiently near the top and/or sides of the single-MLR stack.

In the second category of waveguides and/or resonators, illustratedschematically in transverse-section in FIG. 10, waveguide 1000 comprisesa pair of MLR stacks 1002 and 1006 are employed to confine and guide apropagating optical mode, one from above and one from below (referred tohereinafter as a dual-MLR stack). The two MLR stacks (which may or maynot be substantially similar), as well as a waveguide layer 1004(alternatively, a “core” layer) provided therebetween (along which thepropagating optical mode is substantially confined), are shown in theform of an elongated ridge-like structure protruding from a substrate1010. Lateral confinement of the optical mode may arise in a mannersimilar to that described in the preceding paragraph, either by indexcontrast between the waveguide structure 1000 and a surroundinglower-index medium, or by lateral (uni-lateral and/or bilateral)lower-index portions provided in some or all of the layers of the MLRstacks and/or waveguide layer, which substantially confine the opticalmode within the dual-MLR waveguide structure, as shown schematically intransverse-section in FIG. 14. FIG. 14 shows a waveguide 1400 onsubstrate 1410, waveguide 1400 comprising bottom MLR stack 1402 withlower-index portions 1403, core layer 1404 with lower-index portions1405, and top MLR stack 1406 with lower-index portions 1407. In FIG. 14,one, any two, or all three of MLR stacks 1402 and 1406 and core layer1404 may be provided with respective lower-index lateral portions 1403,1407, and 1405. Lateral confinement may alternatively be provided bylateral metallic coatings, lateral dielectric coatings, lateralmulti-layer reflectors or distributed Bragg reflectors, and/or internalreflection at a waveguide lateral surface. In either FIG. 10 or FIG. 14,evanescent portions of the optical mode may extend laterally from one orboth sides of the waveguide layer, thereby enabling transverse-couplingbetween the optical mode and other optical modes propagatingsufficiently near a side of the dual-MLR stack waveguide structure.

Intermediate between these two categories are structures comprising alower MLR stack, a waveguide or core layer, and a partial upper MLRstack. Such structures shall be referred to herein as partial dual-MLRstack waveguide structures, and are shown schematically intransverse-section in FIGS. 9 and 13. In FIG. 9, waveguide 900 (onsubstrate 910) comprises a lower MLR 902, and core layer 904, and apartial upper MLR 906. In FIG. 13, waveguide 1300 (on substrate 1310)comprises a lower MLR 1302 with lateral lower-index portions thereof1303, core layer 1304 with lateral lower-index portions thereof 1305,and partial upper MLR 1306 with lateral lower-index portions thereof1307. In FIG. 13, any one, any two, or all three of MLR stacks 1302 and1306 and core layer 1304 may be provided with respective lower-indexlateral portions 1303, 1307, and 1305. Such structures provide a rangeof behaviors intermediate between the single-MLR structures (wherein thesupported optical mode has a substantially fully accessible evanescentportion extending upward from the top of the waveguide) and the dual-MLRstructures (wherein substantially no evanescent portion of the supportedoptical mode extends upward from the top of the waveguide). Providing apartial upper MLR stack allows the extent of the evanescent portion ofthe supported optical mode extending upward from the top of thewaveguide to be tailored to fit a particular application by varying thenumber and characteristics of the layers comprising the partial upperMLR stack.

The exemplary transverse waveguide structures illustrated in FIGS. 7-14are shown having layers of the multi-layer reflector stackssubstantially parallel to the substrate and providing confinement of aguided optical mode along a vertical direction. It is also possible andmay be desirable to construct, fabricate, assemble, or otherwise providewaveguides having layers of one or more multi-layer reflector stackssubstantially perpendicular to the substrate, thereby providingconfinement of a guided optical mode along a horizontal direction. Anexample of such a structure is shown in transverse section in FIG. 62A,including MLR stacks 6202 surrounding a core 6206, all positioned onsubstrate 6210. It may be desirable to construct, fabricate, assemble,or otherwise provide waveguides having multi-layer stacks with layers inboth substantially parallel and substantially perpendicularorientations, so as to provide confinement of a guided optical modealong both horizontal and vertical directions. Alternatively, it may bedesirable to provide one or more layers of a multi-layer waveguidestructure with a grating. Such a grating may serve to provide lateralconfinement for a support optical mode, and may also cause the waveguideto exhibit desirable dispersive properties. An example of such awaveguide is shown in transverse section in FIG. 62B positioned on asubstrate 6219. Core layer 6220 is provided with a central portion 6222(along which a guided mode may propagate) and lateral grating portions6224. Upper and lower clad layers 6230 are provided below and, ifdesired, above core layer 6220, and may comprise a single layer oflower-index material or a MLR stack. Upper and lower layers 6230 serveto confine a guided optical mode vertically, while grating portions 6224of core layer 6220 serve to confine the guided optical modehorizontally. Grating portions 6224 may be provided using any suitablespatially-selective material processing techniques.

Part of the utility of MLR-based waveguide structures in waveguidesand/or resonators incorporated into optical devices arises from theirdispersive optical properties, which enable dispersion-engineering ofthe devices. A MLR waveguide exhibits a substantially flat dispersionrelation for guided optical modes over mid-IR, near-IR, and visiblewavelengths, so that a narrow range of wavelengths spans a wide range ofpropagation constants (equivalently, a wide range of modal indices).This may be exploited in a variety of ways. A waveguide incorporating aMLR structure may be used to modal-index-match to another opticalcomponent having a substantially different refractive index. Suchmodal-index-matching may be achieved by appropriate and accurate designand fabrication of the multi-layer reflector (so-called passivemodal-index-matching). Alternatively, an electrical or optical signalmay be applied to a multi-layer reflector incorporating one or moreelectro-active or non-linear-optical layers, respectively, to achievemodal-index-matching over a substantial range of modal-indices(so-called active modal-index-matching).

Waveguide and/or resonator structures as described in the precedingparagraphs may find widely applicable utility in the fields offiber-optic telecommunications and sensors and integrated opticaldevices. Optical power transfer between various optical devices in afiber-optic telecommunications system frequently rely on opticalcoupling between optical modes in the devices. Transverse-coupling maybe employed, thereby eliminating spatial-mode-matching requirementsimposed by end-coupling. Optical signal power transfer bytransverse-coupling depends in part on the relative modal indices of thetransverse-coupled optical modes. Active control of the modal index ofone or both of the transverse-coupled optical modes therefore enablesactive control of the degree to which optical signal power istransferred from one device to the other. If one of the devices to becoupled were to comprise an active waveguide or resonator according tothe present invention, optical power transfer between the devices couldthen be controlled through active control of the modal index asdescribed in the preceding paragraph, typically using control signals ofsubstantially smaller magnitude than required by previously availabledevices. Optical signal power transfer from a fiber-optic or otherlow-index optical waveguide into an integrated on-chip optical device(typically high-index) may be greatly improved and/or activelycontrolled by employing waveguide and/or resonator structures accordingto the present invention. Optical losses within such an integratedon-chip device may be reduced. Waveguides and resonators according tothe present invention may be substantially modal-index-matched tooptical fiber or other low-index waveguides, may possess low opticalloss and/or high optical Q-factors, and may be controlled by relativelysmall control signals. Waveguides according to the present invention maybe used for phase modulation in interferometric optical devices (such asa Mach-Zender interferometer modulator, for example) using smallercontrol signals than required by previously available devices.

A suitable multi-layer reflector (MLR) according to the presentinvention preferably includes a periodic, partially periodic,multi-periodic, quasi-periodic, and/or similar multi-layer dielectricstack. A MLR stack preferably includes layers of varying index(typically alternately increasing and decreasing index; oftenalternating layers of a higher-index material and a lower-indexmaterial) and exhibits wavelength-dependent optical properties.Graded-index materials may also be employed. A distributed Braggreflector (DBR) may serve as a preferred MLR according to the presentinvention and is shown and described in exemplary embodiments disclosedherein. However, other types of multi-layer-reflector structures may beemployed while remaining within the scope of inventive conceptsdisclosed and/or claimed herein.

A distributed Bragg reflector (DBR) preferably includes alternatingquarter-wave (λ/4) layers of dielectric materials having a sufficientlylarge material refractive index differential (typically expressed asΔn/n_(avg)), typically greater than about 8%, preferably greater thanabout 15%, most preferably greater than about 60%. Determination of thequarter-wave thickness depends on the design wavelength and the materialindex of the layer at the design wavelength, and typically a range oflayer thicknesses will function suitably at a given design wavelength.Fabrication techniques for the materials used must enable sufficientlyprecise growth or deposition of substantially uniform layers ofmaterial, typically with nanometer-scale precision. Such fabricationtechniques often require use of materials (often semi-conductors,particularly III-V semiconductors and/or alloys, quantum wells,multi-quantum wells, super-lattices, and/or oxidation products thereof;other suitable materials may be equivalently employed) with crystallinelattice parameters sufficiently similar to allow deposition of layers ofthe different materials on each other without substantial generation ofstrain and/or defects in the materials. Growth or deposition may alsoinvolve amorphous materials. Preferred techniques include as examplescrystalline growth or re-growth, amorphous growth or re-growth, vapordeposition, chemical vapor deposition, epitaxial deposition, beamdeposition, beam-assisted and/or beam-enhanced deposition (beams mayinclude optical, electron, ion, plasma, neutral, radical, and so forth),sputter deposition, plasma and/or ion beam deposition; other suitabletechniques may be equivalently employed. The use of III-Vsemi-conductors and/or alloys thereof for implementation of the presentinvention enables: use of technologically mature deposition, materialprocessing, and other fabrication techniques; attainment of desirableoptical properties for waveguide/resonator devices in the wavelengthregion(s) of interest (including a controllable refractive index viaelectro-active and/or non-linear-optical properties); integration of theoptical device(s) onto a substrate along with control elements thereforand electrical and/or optical connections thereto; integration of theoptical device(s) onto a substrate along with III-V-based light sourcesand/or detectors. Oxidation of III-V semiconductors and/or alloys alsoyields substantially lower-index oxides of high optical quality,enabling fabrication of high-index-contrast MLR stacks andhigh-index-contrast lateral confinement of optical modes therein. All ofthese considerations limit, however, the particular combinations ofmaterials and fabrication techniques that may be employed for a devicesuitable for a given application, which will be described in detailhereinbelow.

A general description of fabrication of MLR waveguides and resonatorsand general considerations dictating choices for materials follows. Avertical layer sequence may typically be constructed first (i.e.“vertical fabrication”) on one or more suitable substrates. The verticalfabrication may proceed as a single sequence of layer depositions on asingle substrate to achieve the desired multi-layer structure (referredto hereinafter as “single-substrate vertical fabrication”).Alternatively, the vertical fabrication may proceed as multiplesequences of layer depositions on multiple substrates, withwafer-bonding and substrate etching techniques employed to form thesingle desired multi-layer structure on a single substrate (referred tohereinafter as “multi-substrate vertical fabrication” or “wafer-bondingvertical fabrication”). It should be noted that “wafer-bonding” shallgenerally encompass any technique suitable for bringing twosubstantially planar materials into substantially intimate contactsubstantially free of voids therebetween and establishing a bondtherebetween. In addition to bringing the two surfaces into contact,such techniques may further involve elevated temperature and or pressurefor periods of time in order to bond the materials together.Single-substrate fabrication may be employed when all layer materials tobe used are sufficiently compatible in their lattice properties to forma sufficiently low-strain and defect-free multi-layer structure by layerdeposition. When insufficiently compatible materials are used,multi-substrate fabrication may be employed to produce a sufficientlylow-strain and defect-free multi-layer structures, which aresubsequently wafer-bonded or otherwise assembled together, perhaps usingpolymer-based or other adhesive. Multi-substrate vertical fabricationtherefore permits a much wider range of material combinations to beemployed, at the expense of more complex fabrication procedures. Thewider range of material combinations may enable, for example, use ofmaterials optimized for forming MLR stacks of high optical quality inconjunction with electro-active and/or non-linear optical materialsoptimized for the design wavelength but that may not belattice-compatible with the MLR materials. The ability to use a widerrange of materials enables tailoring of the electro-optic properties ofthe waveguide and the dispersive properties of the MLR stack(s) forspecific performance characteristics of an optical device employing thewaveguide. It should be noted that multi-substrate fabrication may bedesirable and employed with materials that might also be suitable forsingle-substrate fabrication.

Vertical fabrication is preferably followed by spatially selectiveprocessing of portions of some or all layers of the multi-layerstructure (i.e., “horizontal fabrication”), producing a waveguide orresonator of the desired size, shape, and topology. Horizontalfabrication may include removal of multi-layer material to leaveprotruding ridge, protruding mesa, stepped, and/or recessedstructure(s); such structures may be shallow structures involving onlythe few top layers of the multi-layer structure, or may be deepstructures involving most or all of the layers of the multi-layerstructure. Horizontal fabrication may include deposition of lateralcoatings on such a protruding, stepped, or recessed structures,including but not limited to lateral metallic coatings (opticallyreflective and/or electrical contact layers), lateral dielectriccoatings, lateral multi-layer reflectors or distributed Braggreflectors, or other lateral coating. Horizontal fabrication may alsoinclude chemical conversion and/or modification of some or all layers ofthe MLR or DBR, either after forming the protruding, stepped, orrecessed structure(s), and/or for forming a buried structure. Theconversion/modification may involve all, some, or none of eachindividual layer of the multi-layer structure, and may proceed from oneor both sides of a protruding and/or recessed structure. Horizontalfabrication may include spatially-selective modification of lateralportions of the waveguide structure for lateral confinement of a guidedoptical mode. This may include providing lateral cladding or MLR layersby spatially-selective chemical, physical, or optical modification ofthe multi-layer structure by material deposition or re-deposition,material growth or re-growth, photolithography, beam lithography,doping, implantation, densification, etching, or other suitabletechniques including other material growth/deposition/processingtechniques recited herein.

In addition to MLR layers, waveguide core layers, electro-active layers,and/or non-linear optical layers, additional layers may be included asphysical spacers and/or insulators (i.e., buffer layers), as protectiveoverlayers (i.e., cladding layers), as conductive electrical contacts(i.e., contact layers; metallic and/or semi-conductor), and/or layersfor enabling control of the fabrication processes (i.e., etch-stoplayers). A buffer layer may function as an electrical buffer (moving aportion of the waveguide structure beyond the localized influence of acontact layer, for example) and/or as an optical buffer (maintaining aportion of a guided optical mode away from layers having undesirableoptical properties, adjust the thickness of a particular layer, or otherpurposes). Individual layers may fulfill more than one of thesefunctions. For many of the structures disclosed herein, the waveguidelayer structure may preferably include top and bottom electrical contactlayers with one or more electro-active layers therebetween. Applicationof a voltage across these contact layers produces an electric fieldsubstantially perpendicular to the layers of the waveguide(substantially vertical). This electric field causes variation of theoptical properties any electro-active layers present, and the dispersiveproperties of the MLR structure(s) of the waveguide result in asubstantial change in the modal index of a guided optical mode (asdescribed hereinabove). Other configurations may be employed for theapplication of an electric field to the electro-optic layer(s), however,and substantially horizontal electric fields (transverse and/orlongitudinal) may be employed, for example, without departing frominventive concepts disclosed and/or claimed herein.

For a single-MLR device, a wafer may be grown including a suitablesubstrate, a bottom buffer/etch-stop layer (if desired or needed), adoped bottom electrical contact layer (often delta-doped in the case ofIII-V semiconductors and alloys), another buffer/etch-stop layer (ifdesired or needed), alternating λ/4 layers of a pair of materialscomprising a DBR stack (either having sufficient index contrast to forma DBR, or which may be converted during subsequent processing tomaterials possessing such an index contrast; typically between 3 and 10or more pairs of layers may be employed), a top buffer/etch-stop layer(if desired or needed), a top core layer (between λ/4 and λ/2), and atop cladding/etch-stop layer (if desired or needed). Electro-activeand/or nonlinear optical properties for active control of thewaveguide/resonator may be incorporated into this structure in a varietyof ways, if desired or needed.

In a group of single-substrate fabrication processes for fabricatingsingle-MLR devices, one or more of the λ/4 layers, buffer/etch-stoplayers (if present), core layer, and/or cladding/etch-stop layer maycomprise an electro-active or non-linear-optical material. The flowchartof FIG. 15 and process diagram of FIG. 16 illustrate a single-substratefabrication procedure wherein the core layer 1604 comprises anelectro-active or non-linear-optical material. First, a doped electricalcontact/etch-stop layer 1620 (meaning that the contact layer may alsofunction as an etch-stop layer, if desired) is deposited on substrate1610 (if desired or needed). Buffer/etch-stop layers (meaning theselayers may function as buffers and/or etch-stop layers) may optionallybe deposited before and/or after deposition of contact layer 1620. Inthis and all succeeding diagrams, optional buffer/etch-stop layers andcladding/etch-stop layers are omitted for clarity. A DBR stack 1602 ofabout 5 to about 20 alternating lower- and higher-index quarter-wavelayers is then deposited, with the topmost layer comprising alower-index layer. “Lower-index” and “higher-index” here describe thematerial indices of the layers as they will exist after all fabricationand processing are complete, as does the description “quarter-wave”. Forexample, the proper quarter-wave thickness for a particular layer is notnecessarily determined by the material index of the material deposited,but the material which eventually comprises the layer. In some casesthis will be the deposited material, but in other cases may be a newmaterial obtained from the deposited material through a chemicalconversion process during subsequent processing. Similarly, whether agiven layer is a lower-index or higher-index layer depends on the indexof the layer material after all fabrication and processing are complete.A higher-index deposited material may, for example, be converted to alower-index layer material during subsequent processing. A waveguidecore layer 1604 may then be deposited (after an optionalbuffer/etch-stop layer, if desired) comprising a higher-indexelectro-active or non-linear-optical material and having a layerthickness between about quarter-wave and about half-wave. Thedistinction between a quarter-wave core layer 1604 and the quarter-wavelayers of DBR 1602 is a somewhat artificial one. A top doped electricalcontact/etch-stop layer 1630 may then be deposited (if desired orneeded; preceded by an optional buffer/etch-stop layer if desired, andfollowed by an optional cladding/etch-stop layer if desired), completingthe vertical fabrication of this particular structure.

In the layer deposition scheme described hereinabove and in otherdeposition schemes described hereinbelow, it should be noted thatelectrical contact layers may only be required if electro-activelayer(s) are included in the multi-layer waveguide structure (typicallyfor active modal-index control). Such electrical contact layers and/orelectro-active layers may be omitted from waveguides incorporating oneor more non-linear-optic layers for active modal-index control or fromwaveguides employing passive modal-index matching. While top and bottomelectrical contact layers may preferably be located near the top andbottom, respectively, of the multi-layer structure, this need not alwaysbe the case. Top and bottom electrical contact layers may be placed inany suitable position within the multi-layer structure with theelectro-active layer(s) therebetween. Electrical contact layers maypreferably be oriented substantially parallel to the other layers in themulti-layer structure, so that an electric field applied through thecontact layers would be substantially perpendicular to the layers.Alternatively, electrical contacts may be applied laterally so that acontrol electric field would be applied substantially parallel to thelayers of the multi-layer structure. It should also be noted that whilethe layer deposition schemes recited herein describe deposition of DBRstacks, all of these deposition schemes may be generalized to includeany MLR structure, including periodic, partially periodic,multi-periodic, quasi-periodic, or other varying-index MLR structure,while remaining within the scope of inventive concepts disclosed and/orclaimed herein.

The flowchart of FIG. 17 and process diagram of FIG. 18 illustrate asingle-substrate vertical fabrication procedure wherein a layer 1808 ofelectro-active or non-linear-optical material is deposited separatelyfrom core layer 1804. First, a doped electrical contact/etch-stop layer1820 is deposited on substrate 1810. Buffer/etch-stop layers mayoptionally be deposited before and/or after deposition of contact layer1820, and are not shown. A DBR stack 1802 of about 5 to about 20alternating lower- and higher-index quarter-wave layers is thendeposited, with the topmost layer comprising a lower-index layer. Awaveguide core layer 1804 may then be deposited (after an optionalbuffer/etch-stop layer, if desired) comprising a higher-index materialand having a layer thickness between about quarter-wave and abouthalf-wave. Electro-active or non-linear-optical material layer 1808 maythen be deposited (preceded by an optional buffer/etch-stop layer ifdesired). A top doped electrical contact/etch-stop layer 1830 may thenbe deposited (preceded by an optional buffer/etch-stop layer if desired,and followed by an optional cladding/etch-stop layer if desired),completing the vertical fabrication of this particular structure.Alternatively, the order of deposition of the core layer andelectro-active or non-linear-optical layer may be reversed.

The flowchart of FIG. 19 and process diagram of FIG. 20 illustrate asingle-substrate vertical fabrication procedure wherein a materialcomprising at least one layer of DBR stack 2002 is an electro-active ornon-linear-optical material. First, a doped electrical contact/etch-stoplayer 2020 is deposited on substrate 2010. Buffer/etch-stop layers mayoptionally be deposited before and/or after deposition of contact layer2020, and are not shown. A DBR stack 2002 of about 5 to about 20alternating lower- and higher-index quarter-wave layers is thendeposited, with the topmost layer comprising a lower-index layer. One ormore of the DBR layers (lower-index material, higher-index material, orboth DBR materials) may comprise a layer of electro-optic active ornon-linear-optical material. A waveguide core layer 2004 may then bedeposited (after an optional buffer/etch-stop layer, if desired)comprising a higher-index material and having a layer thickness betweenabout quarter-wave and about half-wave. A top doped electricalcontact/etch-stop layer 2030 may then be deposited (preceded by anoptional buffer/etch-stop layer if desired, and followed by an optionalcladding/etch-stop layer if desired), completing the verticalfabrication of this particular structure. Direct incorporation ofelectro-active or non-linear-optical material into DBR 2002 enablessimplification of the vertical fabrication of an active waveguideaccording to the present invention.

In each of the vertical layer structures of FIGS. 15-20 fabricated usingsingle-substrate vertical fabrication procedures, the lattice propertiesof the electro-active or non-linear-optical material must besubstantially compatible with those of the λ/4 layers (DBR layers), corelayer, the upper electrical contact layer, and/or upper cladding layer(if present), in order to form a sufficiently low-strain and/ordefect-free structure. Application of a control voltage across the topand bottom contact layers (vertical control electric field) enablesactive control of the optical properties of an electro-active layer, inturn enabling control of a modal index of an optical mode supported bythe ultimate waveguide structure. Alternatively, the top and bottomelectrical contact layers may be omitted and replaced with lateralelectrical contacts during subsequent horizontal fabrication, enablingapplication of a horizontal control electric field. Application of acontrol optical signal enables active control of the optical propertiesof a non-linear optical layer, in turn enabling control of a modal indexof an optical mode supported by the ultimate waveguide structure.

If the lattice properties of the DBR materials and the electro-active ornon-linear-optical material are not substantially compatible, a group ofmulti-substrate vertical fabrication processes may be employed toconstruct surface-guided waveguides. The flowchart of FIG. 21 andprocess diagram of FIG. 22 illustrate a two-substrate verticalfabrication procedure wherein two separate substrates may be utilizedfor deposition of material layers and the resulting structures may bewafer-bonded together, eliminating the need for substantially compatiblelattice properties. First, a doped electrical contact/etch-stop layer2220 is deposited on a first substrate 2210. Buffer/etch-stop layers mayoptionally be deposited before and/or after deposition of contact layer2220, and are not shown. A DBR stack 2202 of about 5 to about 20alternating lower- and higher-index quarter-wave layers is thendeposited, with the topmost layer comprising a lower-index layer. Awaveguide core layer 2204 may then be deposited (after an optionalbuffer/etch-stop layer, if desired) comprising a higher-index materialand having a layer thickness between about quarter-wave and abouthalf-wave. A second doped electrical contact/etch-stop layer 2230 isdeposited on a second substrate 2240, which need not belattice-compatible with substrate 2210 or any of the layers depositedthereon. Buffer/etch-stop layers may optionally be deposited beforeand/or after deposition of contact layer 2230, and are not shown.Electro-active or non-linear-optical material layer 2208 may then bedeposited (followed by an optional buffer/etch-stop layer if desired).The second substrate is then inverted, and wafer bonded to the topmostlayer on the first substrate using any suitable wafer-bonding technique.In this way materials having lattice properties insufficientlycompatible to allow direct deposition of a single multi-layer structuremay nevertheless be incorporated into such a structure. Afterwafer-bonding, the second substrate 2240 may be etched away, completingthe vertical fabrication of this particular structure.

Several related alternative two-substrate vertical fabricationprocedures are illustrated in FIGS. 23, 24, 25, and 26. In FIGS. 23 and24, a doped electrical contact/etch-stop layer 2420 is first depositedon a first substrate 2410. Buffer/etch-stop layers may optionally bedeposited before and/or after deposition of contact layer 2420, and arenot shown. A DBR stack 2402 of about 5 to about 20 alternating lower-and higher-index quarter-wave layers is then deposited, with the topmostlayer comprising a lower-index layer, after which an optionalbuffer/etch-stop layer may be deposited, if desired. A second dopedelectrical contact/etch-stop layer 2430 is deposited on a secondsubstrate 2240, which need not be lattice-compatible with substrate 2410or any of the layers deposited thereon. Buffer/etch-stop layers mayoptionally be deposited before and/or after deposition of contact layer2430, and are not shown. A waveguide core layer 2404 may then bedeposited (followed by an optional buffer/etch-stop layer, if desired)comprising a higher-index material and having a layer thickness betweenabout quarter-wave and about half-wave. Electro-active ornon-linear-optical material layer 2408 may then be deposited (followedby an optional buffer/etch-stop layer if desired). The second substrateis then inverted, and wafer bonded to the topmost layer on the firstsubstrate using any suitable wafer-bonding technique. Afterwafer-bonding, the second substrate 2440 may be etched away, completingthe vertical fabrication of this particular structure. The order ofdeposition of the core layer and electro-active or non-linear-opticallayer may be reversed. FIGS. 25 and 26 illustrate an analogousfabrication procedure involving a first substrate 2610 with contactlayer 2620 and DBR 2602, and a second substrate 2640 with contact layer2630 and electro-active or non-linear-optical core layer 2604.Wafer-bonding the topmost layers of the two substrates and then etchingaway substrate 2640 yields the desired layer structure.

Whichever class of procedures is used (single-substrate, ortwo-substrate), the resulting vertical structure comprises a single DBRstack for surface guiding an optical mode and incorporates at least oneelectro-active or non-linear-optical layer, with contact layers aboveand below (if needed). Application of a control voltage across the topand bottom electrical contact layers (vertical control electric field)enables active control of the optical properties of the electro-activelayer. In either group of surface-guided structures, the top and bottomelectrical contact layers may be omitted and replaced with lateralelectrical contacts during subsequent horizontal fabrication, enablingapplication of a horizontal control electric field. Application of acontrol optical signal enables active control of the optical propertiesof a non-linear optical layer.

For devices incorporating two MLR structures confining an optical modetherebetween (i.e., dual- or partial-dual-DBR devices), similar single-and multi-substrate vertical fabrication methods may be employed. Awafer may be grown comprising a suitable substrate, a bottom buffer orcladding layer (if desired or needed), a doped bottom electrical contactlayer (often delta-doped in the case of III-V semiconductors andalloys), another buffer or cladding layer (if desired or needed), and afirst set of alternating λ/4 layers of a pair of materials comprising abottom DBR stack (either having sufficient index contrast to form a DBR,or which may be converted during subsequent processing to materialspossessing such an index contrast; typically between 3 and 10 or morepairs of layers may be employed). A λ/2 waveguide layer (i.e., corelayer), a top DBR stack of alternating λ/4 layers, and theelectro-active or non-linear-optical properties required for activecontrol of the waveguide/resonator may be incorporated into thisstructure in a variety of ways.

The flow diagram of FIG. 27 and fabrication process diagram of FIG. 28show a single-substrate vertical fabrication process wherein a bottomcontact layer 2820 is deposited on a substrate 2810 (preceded and orfollowed by buffer/etch-stop layers if desired or needed; not shown).After deposition of bottom DBR stack 2802 (and a buffer/etch-stop layer,if desired), half-wave waveguide core layer 2804 may be depositeddirectly over the bottom DBR stack 2802 (followed by a buffer/etch-stoplayer, if desired). An electro-active or non-linear-optical layer 2808may be deposited next (followed by a buffer/etch-stop layer, ifdesired). The order of deposition of the core layer and electro-activeor non-linear-optical layer may be inverted. A second set of alternatingλ/4 layers (either the same pair of materials as used for bottom DBRstack 2802, or a suitable alternative pair of substantiallylattice-compatible materials) may then be deposited over electro-activeor non-linear-optical layer 2808 to form top DBR stack 2806. DBR stack2806 may typically comprise from 1 to about 20 layers of alternatinglower- and higher-index quarter-wave layers with a lower-indexbottom-most layer, the number of layers being employed depending on theevanescent properties sought for the waveguide ultimately produced.Fewer layers in top DBR stack 2806 results in a larger evanescentportion of a supported optical mode extending upward from the waveguidebeyond upper DBR stack 2806. A top electrical contact/etch-stop layer2830 (preceded and/or followed by buffer/etch-stop layer(s) if desired)may then be deposited, substantially completing the vertical fabricationof the double DBR layer structure.

Instead of a separate electro-active or non-linear-optical layer 2808,the single-substrate fabrication procedure of FIGS. 29 and 30 may befollowed, comprising deposition on substrate 3010 of contact layer 3020,bottom DBR stack 3002, core layer 3004 comprising a half-wave layer ofelectro-active or non-linear-optical material, top DBR stack 3006, andtop contact layer 3030. Alternatively, as illustrated in FIGS. 31, 32,33, and 34, one or more layers of the bottom DBR 3402 and/or top DBR3406 may comprise a layer of electro-active or non-linear-opticalmaterial. In any of these single-substrate vertical fabricationprocedures the lattice properties of the electro-optic layer materialmust be substantially compatible with those of the λ/4 layers, waveguidelayer, and or contact layers in order to form a sufficiently low-strainand/or defect-free structure. Application of a control voltage acrossthe top and bottom contact layers (vertical control electric field)enables active control of the optical properties of the electro-activelayer, wherever it is located. Application of a control optical signalenables active control of the optical properties of a non-linear opticallayer.

If the lattice properties of the various layer materials are notsufficiently compatible to enable vertical fabrication of dual- orpartial-dual-DBR structures as a single growth sequence on a singlesubstrate as described above, multi-substrate vertical fabricationtechniques may be employed to enable incorporation oflattice-incompatible materials into dual- or partial-dual-DBRwaveguides. In each of the fabrication procedures illustrated in FIGS.35, 36, 37, 38, 39, and 40 three substrates are employed. A bottomcontact and bottom DBR stack are grown on a first substrate, anelectro-active or non-linear-optical is deposited on a second substrate(alone, along with a separate core layer, or as the core layer), and atop contact and top DBR stack are grown on a third substrate. If awaveguide layer is not deposited on the second substrate, it may bedeposited on either of the other substrates. The second substrate (withthe electro-active or non-linear-optical layer) is inverted andwafer-bonded to the first substrate (with the bottom DBR stack), and thesecond substrate is etched away. The third substrate (with the top DBRstack) is then inverted and wafer-bonded to the remaining layers of thesecond substrate. The third substrate is removed, substantiallycompleting the vertical fabrication of the dual-DBR layer structure.With these procedures the lattice properties of the electro-active ornon-linear-optical material need not be compatible with those of eitherset of λ/4 layers. Application of a control voltage across the top andbottom electrical contact layers (vertical control electric field)enables active control of the optical properties of the electro-activeor non-linear-optical layer. In either group of dual-DBR structures(single-substrate or multi-substrate vertical fabrication), the top andbottom electrical contact layers may be omitted and replaced withlateral electrical contacts during subsequent horizontal fabrication,enabling application of a horizontal control electric field. Applicationof a control optical signal enables active control of the opticalproperties of a non-linear optical layer.

Whichever of the above described vertical fabrication methods isemployed (single- or multi-substrate), and whether a single- or dual-DBRstructure is employed, the resulting multi-layer structure must befurther processed (by so-called “horizontal fabrication”) to producelaterally-confined waveguides and resonators according to the presentinvention. Such waveguides and resonators may take the form of aprotruding ridge-like structure, a protruding mesa-like structure, astepped structure, a recessed structure, and/or a buried structure onthe substrate. Alternatively, such waveguides and resonators may takethe form a structures of varying density, chemical composition,refractive index, or other physical property. These structures may takethe form of linear segments, arcuate segments, or other open waveguidestructures, or may take the form of rings, circles, ovals, racetracks,ellipses, polygons, or other closed waveguide (i.e., resonator)structures. Other topologies may be employed for more specializedintegrated optical devices, such as Mach-Zender interferometers,directional couplers or 2×2 switches, and the like (as in FIGS. 1 and 2,for example). Once a wafer has been produced according to any of thevertical fabrication methods described hereinabove (or other suitablyequivalent methods), any suitable spatially-selective lithographicpatterning and/or etching technique(s), or other functionally equivalentspatially-selective material processing techniques, may be employed tomodify portions of the multi-layer structure, thereby forming on thesubstrate structures_(protruding, recessed, buried, chemically orphysically altered, etc) of the desired topology. Generic examples areshown schematically in FIGS. 41A through 43B using apatterned-mask/etching technique. Other suitable techniques, includingdirect lithographic techniques requiring no mask, optical lithographictechniques, deposition, assisted deposition, re-growth, re-deposition,and other techniques and/or processes described herein, for example, maybe equivalently employed.

In FIG. 41A, a substrate 4110 is shown with a multi-layer structure 4160thereon and a mask layer 4170. Mask 4170 may be deposited and spatiallypatterned by any suitable technique, including but not limited tolithographic techniques. The spatial pattern of mask 4170 is determinedby the size, shape, and topology desired for the waveguide or resonatorto be produced, and in this example mask 4170 is configured to yield asimple linear waveguide segment 4100. The un-masked portions ofmulti-layer structure 4160 may be substantially completely removed byany suitable technique, including but not limited to dry and/or wetetching techniques. After removal of the un-masked portions ofmulti-layer 4160, mask 4170 may be removed, leaving waveguide 4100 onsubstrate 4110. Analogous procedures are illustrated in FIGS. 42A and43A involving substrate 4210/4310, multi-layer 4260/4360, and mask4270/4370 yielding resonator 4200/4300. It should be noted that asmulti-layer 4160/4260/4360 is removed, laterally exposed portions of themulti-layer may come under attack during some etching procedures, andthe size and shape of the resulting waveguide 4100/4200/4300 may bedifferent than the initial size and shape of mask 4170/4270/4370(slightly smaller and/or narrower, for example). Depending on theprecise nature and sequence of layers of multi-layer 4160/4260/4360, andthe presence/absence of etch-stop layers therein, more complex, steppedstructures may be obtained. This may be advantageous for leavingportions of contact layers exposed for later electrical connection to acontrol signal source, for example, or for producing localized contactlayers for applying localized control signals, or for producing awaveguide/resonator 4100/4200/4300 having optical properties that varyalong its length, or for other purposes. It may be desirable to performa series of deposition and/or wafer-bonding steps alternating withspatially-selective etch steps (i.e., intermingling the “verticalfabrication” and the “horizontal fabrication”), to obtain complexwaveguide/resonator structures.

The multi-layer structure may be deep-etched (i.e., most or all of theway through the multi-layer structure down toward the substrate; FIGS.41A, 42A, and 43A). In this case modes supported by the waveguide may bestrongly laterally confined by the relatively large index contrast atthe sides of the waveguide. Alternatively, a relatively shallow etch maybe employed (FIGS. 41B, 42B, and 43B), removing material from only thetop few layers (or even a portion of only one layer). The lateraloptical confinement provided by such a shallow-etched waveguide iscorrespondingly weaker than that provided by a deep-etched waveguide.This may provide desirable optical performance characteristic for theresulting waveguide, such as support of fewer transverse optical modesthan a deep-etched waveguide or reduced optical loss induced by etchedsurfaces, for example. The two sides of waveguide/resonator4100/4200/4300 may each have material removed to the same depth, or todiffering depths, as desired for fabricating specific devices.

The index contrast between the sides of the waveguide structure and thesurrounding lower-index medium (examples given hereinabove) may providelateral confinement of an optical mode supported by thewaveguide/resonator structure (whether single-, dual-, orpartial-dual-MLR). However, the etched side surfaces will often havesubstantial roughness and/or numerous defects due to the etchingprocess, degrading the propagation characteristics and/or mode qualityof a supported optical mode and/or degrading the Q-factor of an opticalresonator. Also, the relatively large index contrast may give rise toundesirable multi-transverse-mode behavior. It may therefore bedesirable to provide some or all of the layers of the multi-layerstructure (either single- or dual-MLR structures) with laterallower-index portions having a refractive index intermediate between thehigher-index of the medial portion of the respective layer and the indexof the surrounding medium (usually air, but possibly some other ambientover-layer). This may provide several advantages, including: 1) asupported optical mode may be confined by the index contrast between thehigher-index medial portion and the lower-index lateral portions of thelayers, thereby reducing and/or limiting the number of transverseoptical modes supported by the waveguide and simplifying design andoperation of devices incorporating the waveguide; 2) since it is guidedby the higher-index medial portion of the waveguide, the supportedoptical mode may interact less with the etched lateral surfaces of thewaveguide, thereby limiting the degradation produced by roughness and/ordefects at the etched surface; 3) the processing required to provided alayer with lateral lower-index portions may also reduce the roughnessand/or defect density at the etched lateral surface of the waveguide. Itmay be desirable to extend these lateral intermediate-index portionsacross the entire width of some layers of the multi-layer waveguidestructure, to provide enhanced index contrast in the MLR stack(s).Greater index contrast in the MLR stacks may result in better verticalguiding/confinement of a supported optical mode using fewer layers.

Lateral lower-index portions may be readily provided in multi-layerwaveguide structures (both single- and dual-MLR) fabricated using III-Vsemi-conductors and/or alloys, quantum wells, multi-quantum wells,and/or super-lattices thereof. These materials typically have indicesbetween about 2.9 (AlAs) and about 3.4 (GaAs), with Al_(x)Ga_(1-x)Asalloys falling between these extremes. III-V materials havingsubstantial aluminum content may be readily oxidized to aluminum oxides(Al_(x)O_(y)), having indices between about 1.5 and about 1.7. This maybe exploited, for example, by fabricating a MLR stack from alternatinglayers of GaAs 4420 and high-aluminum AlGaAs 4430(Al_(0.98)Ga_(0.02)As), as shown in FIG. 44. After vertical andhorizontal fabrication to produce a protruding_DBR waveguide structure4400, the wafer may be oxidized by (for example) bubbling N₂ throughwater at 85° C. and then passing the N₂ over the waveguide in a furnaceat 425° C. The aluminum-containing layers 4430 are preferentiallyoxidized at a rate of about 1 μm/min (depending on layer thickness,aluminum content, and so forth), and the oxidation proceeds from theexposed edge of each aluminum-containing layer 4430 inward (processreferred to hereinafter as “lateral oxidation”). Depending on theoxidation time, layer thickness, layer aluminum content, and so forth,the oxidized layer may have lateral aluminum oxide portions 4432surrounding a central AlGaAs portion 4434. If the oxidation is permittedto proceed long enough, the entire layer may be converted to an aluminumoxide layer 4436, providing a much higher material index contrast MLR(about 1.5 to 3.4) than the original MLR layer structure (about 2.9 to3.4). It should be noted that lateral oxidation or other lateralchemical modification of any MLR layers may proceed form one or bothsides of a waveguide structure.

It should be noted that the desired thickness of layers 4430 (AlGaAs)depends on whether the oxidation is used to produce lateral oxideportions 4432 or full oxide layers 4436. If lateral oxide portions 4432are to be produced, then the desired quarter-wave thickness for medialportions 4434 is determined based on the design wavelength and materialindex for the AlGaAs alloy being used, and this thickness is providedduring vertical fabrication of the wafer from which MLR 4400 is made. Iffull aluminum oxide layers 4436 are to be produced, the desiredquarter-wave thickness is determined based on the design wavelength andthe material index of the aluminum oxide. This oxide-index-basedthickness is provided for the AlGaAs layers 4430 during verticalfabrication of the wafer. As an example, at a design wavelength of about1500 nm, AlGaAs layers 4430 should be about 130 nm thick to yieldquarter-wave medial AlGaAs portions 4434, but should be about 270 nmthick to yield quarter-wave aluminum oxide layers 4436.

FIG. 45 shows a further refinement of the lateral oxidation schemeoutlined above, wherein the Al concentration of AlGaAs layers 4430varies, decreasing with each additional AlGaAs layer 4430 added duringvertical fabrication of the wafer. The lateral oxidation rate increaseswith increasing Al content, so that for a given oxidation time thevertically tapered MLR of FIG. 45 results, which may or may not includeone or more full oxide layers 4436. (In general, the lateral oxidationrate depends on the thickness of the layer, the chemical composition ofthe surrounding layers, and the Al content. However, in the presentcircumstances, only the Al content can be independently varied.) FIGS.46 and 47 show dual-MLR structures analogous to FIGS. 44 and 45,respectively. The transverse waveguide geometries shown in FIGS. 44through 47 each have desirable optical characteristics. The medialAlGaAs/GaAs MLR's of FIGS. 43-47 have the advantage of horizontallyconfining and guiding a supported optical mode away from lateral edgesof waveguide 4300 and any roughness and/or defects thereon. The higherindex contrast of the GaAs/Al_(x)O_(y) MLR's of FIGS. 44 and 46 enablesvertical confinement and guiding of a supported optical mode using fewerMLR layers. The vertically tapered AlGaAs/GaAs MLR's of FIGS. 45 and 47may better serve to horizontally confine and guide the optical mode.

Both improved horizontal confinement (away from potentially poor opticalquality lateral waveguide surfaces), and vertical confinement with fewerlayers of a higher contrast MLR, may be achieved simultaneously in awaveguide structure. As in the process of FIGS. 45 and 47, differentialoxidation rates may be exploited to achieve various desired transverselayer geometries. As before, a given layer thickness provided duringvertical fabrication of a wafer is determined by the design wavelengthand the index of the material that eventually comprises the layer, notnecessarily the index of the material deposited. In general, oxidationrates of III-V semi-conductors increase with increasing aluminumcontent. A MLR wafer may be fabricated from alternating layers ofAl_(0.98)Ga_(0.02)As 4530 and Al_(0.96)Ga_(0.04)As 4520 (FIG. 48).Following horizontal processing to form a protruding structure 4500,lateral oxidation may be initiated and permitted to proceed until eachentire Al_(0.98)Ga_(0.02)As layer 4530 has been converted to asubstantially complete aluminum oxide layer 4536, while a medial portion4524 of each Al_(0.96)Ga_(0.04)As layer 4520 remains, flanked by lateralaluminum oxide portions 4522. The resulting waveguide structure thencomprises a high-index-contrast central DBR portion(Al_(0.96)Ga_(0.04)As/Al_(x)O_(y); about 2.9 to about 1.5) surroundedlaterally by a lower-index medium (Al_(x)O_(y); about 1.5). As theoxidation of layers 4520 and 4530 progresses, medial portion 4524 maycome under attack and begin to oxidize from above and below. Someexperimentation may be required to determine, for a given set of layercompositions and oxidation conditions, the appropriate thicknesses forlayers 4520 and 4530 to achieve the desired thicknesses for layers 4536and 4524. Other material combinations may be amenable to a schemesimilar to that of FIG. 48. Layers 4520 and 4530 may comprisequarter-wave layers of AlAs/InAs superlattice material, for example,with the AlAs fraction of layers 4530 being higher than the AlAsfraction of layers 4520. The after horizontal fabrication to form ridgewaveguide 4500, lateral oxidation may be employed to produce Al_(x)O_(y)layers 4536 (from substantially complete oxidation of layers 4530)alternating with layers having AlAs/InAs superlattice medial portion4524 and lateral Al_(x)O_(y) portions 4522. During vertical fabricationof the wafer, the thicknesses provided for layers 4520 and 4530 arechosen to yield the desired thicknesses for layers 4536 and 4524. Forboth of these schemes (and functionally equivalent schemes using othermaterial combinations), the lower-index aluminum oxide lateral portions4522 of the resulting DBR waveguide laterally confine a supportedoptical mode away from the lateral surfaces of waveguide 4500, while thehigh index contrast of medial AlAs/InAs portions 4524 and aluminum oxidelayers 4536 provide vertical confinement with fewer DBR layers.

It may be desirable to provide asymmetric lateral lower-index portionsof layers of a waveguide. This may be the case, for example, when adual-DBR waveguide will be used for transverse-coupling to anotheroptical element on only one side of the waveguide. As shown in FIGS. 49and 50, wider lower-index lateral portions 4622 may be provided on thenon-coupling side of the waveguide 4600, thereby reducing orsubstantially eliminating any evanescent portion of a waveguide opticalmode extending beyond the non-coupling side of the waveguide. Narrowerlateral portions 4624 may be provided on the coupling side of thewaveguide 4600, thereby enabling an evanescent portion of an opticalmode guided by medial higher-index medial portions 4620 to extend beyondthe coupling side of waveguide 4600. The differing widths may beachieved by masking the coupling side of waveguide 4600 during a portionof the lateral oxidation process, reducing the extent to which theoxidation progresses across the layers from the coupling side ofwaveguide 4600.

It may be desirable to ensure that the lateral oxidation proceeds fromone side of the waveguide only, so as to avoid material defects that mayarise when counter-propagating oxidation fronts meet within a waveguidestructure along a boundary layer or interface. A shallow etch may beperformed to provide lateral optical confinement for the waveguidestructure. A deeper etch may be done farther away (i.e., far enough soas to substantially eliminate interaction between the supported opticalmode and the deep-etched side surface). Lateral oxidation may thenproceed from the deep-etched side across the waveguide in only onedirection, with no boundary layer or interface being formed.

Specific examples of combinations of materials for fabricating activeoptical waveguides and resonators will now be discussed, along withadvantages and limitations of each and wavelength ranges over which eachmight be suitable. Each combination may be used to fabricate single-,dual-, and/or partial dual MLR waveguides and resonators. Some materialcombinations may be suitable for both single- and multi-substratevertical fabrication, while others may only be suitable formulti-substrate vertical fabrication. These examples may be preferredcombinations for particular uses and/or applications, but should not beconstrued as limiting the scope of inventive concepts disclosed and/orclaimed herein. Other combinations of materials satisfying the generalstructural and functional criteria set forth herein may be employedwithout departing from inventive concepts disclosed and/or claimedherein.

Preferred electro-active (i.e., electro-absorptive and/or electro-optic)or non-linear-optical materials for use in waveguides and resonatorsaccording to the present invention may be quantum-well (QW) andmulti-quantum-well (MQW) materials. A quantum well typically comprises athin layer of a lower bandgap material sandwiched between barrier layersof a higher bandgap material. Thin is defined here as sufficiently thinthat the effective bandgap of the quantum well differs from the bulkbandgap of the lower bandgap material due to spatial confinementeffects, and typical quantum well layers may be on the order of 1-20 nmthick. The optical properties of such quantum wells may be tailored to acertain degree by the composition of the materials used (selected forbandgap, index, etc.), and may be actively controlled by application ofa control electric field to a greater degree than bulk semiconductormaterials. In particular, a quantum well may function as anelectro-absorptive and/or an electro-optic material, via thequantum-confined Stark effect (QCSE), the Franz-Keldysh effect (FKE),the quantum-confined Franz-Keldysh effect (QCFKE), and/or othermechanisms. The use of multiple quantum well layers separated by barrierlayers (on the order of tens of nanometers thick) yields amulti-quantum-well material, wherein the electro-absorptive and/orelectro-optic properties of the individual quantum well layers areadditive. The quantum well and barrier layers are sufficiently thin thatfor optical wavelengths typically used in the waveguides and resonatorsof the present invention, the optical mode behaves substantially as ifthe multi-quantum-well layer were a uniform layer having an index equalto the average index of the layers of the multi-quantum well.

An exemplary waveguide or resonator according to the present inventionmay include a MLR stack(s) comprising alternating quarter-wave layers ofGaAs (index of about 3.5) and Al_(x)Ga_(1-x)As (index between about 2.9and 3.4). The aluminum fraction x may lie between about 0.8 and 1.0,preferably between about 0.9 and 1.0, most preferably between about 0.92and about 0.98. The fabrication of high optical quality DBR stacks withthis material combination is technologically mature and wellcharacterized. A core layer may preferably comprise GaAs, InGaAs, orAlGaAs, and doped GaAs, InGaAs, or AlGaAs may preferably be used forelectrical contact layers. Either p-type of n-type doping may be usedfor the contact layers, and delta doping may be preferred. Buffer,cladding, and/or etch-stop layers, if present, may preferably compriseGaAs, and the waveguide may preferably rest on a GaAs substrate (whichmay possibly be doped to serve as the bottom contact layer). Othersuitable materials may be equivalently employed for the substrate and/orthe core, buffer, cladding, and/or etch-stop layers.

For a single-substrate vertical fabrication of a single- or dual-MLRdevice using GaAa/AlGaAs MLR stack(s), a multi-quantum-well materialcomprising GaAs quantum well layers and Al_(x)Ga_(1-x)As barrier layersmay be employed as the electro-active material. This MQW material islattice-compatible with the GaAs/AlGaAs MLR stack(s), thereby enablingsingle-substrate vertical fabrication. The wavelength range over whichthe useful electro-absorptive and/or electro-optic properties of thisMQW material may extend is from about 0.7 μm to about 0.8 μm, whichdetermines the possible design wavelengths for the waveguide and thecorresponding quarter-wave thicknesses for the MLR stack layers. The MQWmaterial may comprise the entire core layer (FIGS. 15-16 and 29-30), ormay comprise a separate layer (FIGS. 17-18 and 27-28). Waveguides ofthese compositions may be further processed by lateral oxidation of theAlGaAs layers, as shown in FIGS. 44-47 and 49, thereby providing lateralaluminum oxide portions having a lower index (about 1.5-1.7) than themedial AlGaAs portions and confining a supported optical mode away fromthe lateral edges of the waveguide. Permitting lateral oxidation toproceed until substantially complete oxidation of the AlGaAs MLR stacklayers (FIGS. 44, 46 and 50) results in a higher index contrastGaAs/Al_(x)O_(y) MLR stack (about 3.4 to about 1.5). In this and othercases where an entire quarter-wave layer is converted by lateraloxidation, the quarter-wave layer thickness for the initial materialdeposited must be determined based on the index of the final materialpresent after lateral processing (oxidation or otherwise).

For a single-substrate vertical fabrication of a single- or dual-MLRdevice using GaAa/AlGaAs MLR stack(s), a multi-quantum-well materialcomprising GaAs quantum well layers and Al_(x)Ga_(1-x)As barrier layersmay be employed as the electro-active material in place of one or morelayers of a MLR stack. This MQW material is lattice-compatible with theGaAs/AlGaAs MLR stack(s), thereby enabling single-substrate verticalfabrication. The wavelength range over which the usefulelectro-absorptive and/or electro-optic properties of this MQW materialmay extend is from about 0.7 μm to about 0.8 μm, which determines thepossible design wavelengths for the waveguide and the correspondingquarter-wave thicknesses for the MLR stack layers. The MQW material maycomprise one or more layers of the MLR stack(s) (FIGS. 19-20 and 31-34).Waveguides of these compositions may be further processed by lateraloxidation of the AlGaAs and/or GaAs/AlGaAs MQW layers, as shown in FIGS.44-47 and 49, thereby providing lateral aluminum oxide portions having alower index (about 1.5-1.7) than the medial AlGaAs and/or MQW portionsand confining a supported optical mode away from the lateral edges ofthe waveguide. Permitting lateral oxidation to proceed untilsubstantially complete oxidation of some of the MLR stack layers (FIGS.44, 46, and 50) results in a higher index contrast GaAs/Al_(x)O_(y) MLRstack (about 3.4 to about 1.5). In this and other cases where an entirequarter-wave layer is converted by lateral oxidation, the quarter-wavelayer thickness for the initial material deposited must be determinedbased on the index of the final material present after lateralprocessing (oxidation or otherwise).

For a single-substrate vertical fabrication of a single- or a dual-MLRdevice using GaAa/AlGaAs MLR stack(s), a multi-quantum-well materialcomprising GaAs or AlGaAs barrier layers and In_(x)Ga_(1-x)As quantumwell layers may be employed as the electro-active material. This MQWmaterial is lattice-compatible with the GaAs/AlGaAs MLR stack(s),thereby enabling single-substrate vertical fabrication. The wavelengthrange over which the useful electro-absorptive and/or electro-opticproperties of this MQW material may extend is from about 0.9 μm to about1.1 μm, which determines the possible design wavelengths for thewaveguide and the corresponding quarter-wave thicknesses for the MLRstack layers. The MQW material may comprise the entire core layer (FIGS.15-16 and 29-30), or may comprise a separate layer (FIGS. 17-18 and27-28). Waveguides of this configuration may be further processed bylateral oxidation of the AlGaAs layers, as shown in FIGS. 44-47 and 49,thereby providing lateral aluminum oxide portions having a lower index(about 1.5-1.7) than the medial AlGaAs portions and confining asupported optical mode away from the lateral edges of the waveguide.Permitting lateral oxidation to proceed until substantially completeoxidation of the AlGaAs MLR stack layers (FIGS. 44, 46, and 50) resultsin a higher index contrast GaAs/Al_(x)O_(y) DBR stack (about 3.4 toabout 1.5). In this and other cases where an entire quarter-wave layeris converted by lateral oxidation, the quarter-wave layer thickness forthe initial material deposited must be determined based on the index ofthe final material present after lateral processing (oxidation orotherwise).

For a single-substrate vertical fabrication of a single- or dual-MLRdevice using GaAa/AlGaAs DBR stack(s), a multi-quantum-well materialcomprising GaAs or AlGaAs barrier layers and In_(x)Ga_(1-x)As_(1-y)N_(y)quantum well layers may be employed as the electro-active material. Thefraction x may range between about 0.05 and about 0.30, preferablybetween about 0.1 and about 0.3, and most preferably about 0.15. Thefraction y may range between about 0.001 and about 0.04, preferablyabout 0.02. This MQW material is lattice-compatible with the GaAs/AlGaAsMLR stack(s), thereby enabling single-substrate vertical fabrication.The wavelength range over which the useful electro-absorptive and/orelectro-optic properties of this MQW material may extend is from about1.1 μm to about 1.45 μm (at about y=0.02) and may be extended withfurther development. This wavelength range determines the possibledesign wavelengths for the waveguide and the corresponding quarter-wavethicknesses for the MLR stack layers. The MQW material may comprise theentire core layer (FIGS. 15-16 and 29-30), or may comprise a separatelayer (FIGS. 17-18 and 27-28). Waveguides of this configuration may befurther processed by lateral oxidation of the AlGaAs layers, as shown inFIGS. 44-47 and 49, thereby providing lateral aluminum oxide portionshaving a lower index (about 1.5-1.7) than the medial AlGaAs portions andconfining a supported optical mode away from the lateral edges of thewaveguide. Permitting lateral oxidation to proceed until substantiallycomplete oxidation of the AlGaAs MLR stack layers (FIGS. 44, 46, and 50)results in a higher index contrast GaAs/Al_(x)O_(y) MLR stack (about 3.4to about 1.5). In this and other cases where an entire quarter-wavelayer is converted by lateral oxidation, the quarter-wave layerthickness for the initial material deposited must be determined based onthe index of the final material present after lateral processing(oxidation or otherwise).

A waveguide or resonator according to the present invention may includea MLR stack(s) comprising alternating quarter-wave layers ofAl_(0.96)Ga_(0.04)As (index of about 2.9 to 3.0) and Al_(y)O_(z) (indexbetween about 1.5 and 1.7). The MLR stack layers deposited duringvertical fabrication (FIGS. 15-18 and 27-30) comprise alternating layersof Al_(0.96)Ga_(0.04)As (quarter-wave thickness based on an index ofabout 3.0) and Al_(0.98)Ga_(0.02)As (quarter-wave thickness based on anindex of about 1.6), for example. Other aluminum fractions may beequivalently employed, including AlAs, and the aluminum fraction of thelower-aluminum layers may vary with distance from the substrate,yielding a tapered waveguide structure. The electro-active layer maycomprise any of the MQW materials listed thus far (GaAs/AlGaAs,GaAs/InGaAs, GaAs/InGaAsN). Lateral oxidation of the waveguide proceedsmore rapidly in the AlGaAs MLR layers having the higher Al content. Thelateral oxidation is allowed to proceed just to completion in theAl_(0.98)Ga_(0.02)As MLR layers, thereby leaving medial portions ofAl_(0.96)Ga_(0.04)As between lateral Al_(x)O_(y) portions in theAl_(0.96)Ga_(0.04)As layers (FIGS. 48 and 50). The resulting MLR stackcomprises low-index quarter-wave aluminum oxide layers alternating withlayers having a high-index quarter-wave Al_(0.96)Ga_(0.04)As medialportion surrounded by low-index aluminum oxide lateral portions. A corelayer may preferably comprise one of the electro-active MQW materials,GaAs, or AlGaAs, and doped GaAs or InGaAs may preferably be used forelectrical contact layers. Either p-type of n-type doping may be usedfor the contact layers, and delta doping may be preferred. Buffer,cladding, and/or etch-stop layers, if present, may preferably compriseGaAs or AlGaAs, and the waveguide may rest on a GaAs or AlGaAssubstrate. Other suitable materials may be equivalently employed for thesubstrate and/or the core, buffer, cladding, and/or etch-stop layers.

For operation in the 1.2 μm to 1.7 μm region, InGaAsP MQW material grownon an InP substrate is the best characterized and most technologicallymature material available for use as an electro-optic and/orelectro-absorptive layer. The bulk bandgap of the InGaAsP material maybe varied over this wavelength range by varying the stoichiometry.Quantum well layers about 10 nm thick with a 1.6 μm bulk bandgapseparated by barrier layers about 20 nm thick with a 1.2 μm bulk bandgapmay provide desirable electro-optic and/or electro-absorptive behaviorat an operating wavelength of about 1.5 μm, for example. Other bandgapsand/or layer thicknesses may be equivalently employed. Unfortunately,the lattice properties of InGaAsP are not sufficiently compatible withthose of the GaAs/AlGaAs system to enable single-substrate verticalfabrication of sufficiently low-strain and/or defect-free waveguidestructures. Multi-substrate vertical fabrication may be employed,however, to produce such structures, as illustrate in FIGS. 21-26 and35-40. The MLR stack(s) (GaAs/AlGaAs, GaAs/Al_(x)O_(y), orAlGaAs/Al_(x)O_(y)) may be deposited onto GaAs or equivalentsubstrate(s), for example, while the InGaAsP MQW may be deposited ontoan InP or equivalent substrate. The InGaAsP MQW may be wafer-bonded overthe MLR, and the InP substrate may then be etched away, yielding asingle-MLR structure. A second MLR may be wafer-bonded over the MQWlayer and the corresponding GaAs substrate etched away, yielding adual-MLR structure. In this way the desired wavelength-dependentelectro-optic and/or electro-absorptive properties may be incorporatedinto the waveguide despite the lack of lattice compatibility of therequired materials.

Alternatively, the MLR stack(s) may be fabricated using materials thatare lattice-compatible with the InGaAsP MQW system. A waveguide orresonator according to the present invention may include MLR stack(s)comprising alternating quarter-wave layers of InP (index of about 3.4)and aluminum oxide (index about 1.55 at 1.5 μm). The MLR stack depositedduring vertical fabrication (FIGS. 15-18 and 27-30) initially comprisesalternating layers of InP (quarter-wave thickness based on an index ofabout 3.4) and Al_(x)In_(1-x)As (quarter-wave thickness based on anindex of about 1.55), for example. The aluminum fraction may varybetween about 0.5 and about 1.0, preferably between about 0.8 and about1.0. AlAs/InAs super-lattice material (of substantially the same averagecomposition) may be employed instead of AlInAs. Lateral oxidation of thewaveguide results in substantially complete conversion of the AlInAslayers to aluminum oxide (as in FIGS. 44 and 46), thereby yielding a MLRstack comprising alternating quarter-wave layers of high-index InP andlow-index aluminum oxide. A waveguide core layer may preferably compriseInP, and doped InGaAs or InGaAsP may preferably be used for electricalcontact layers. Either p-type of n-type doping may be used for thecontact layers, and delta doping may be preferred. Buffer, cladding,and/or etch-stop layers, if present, may preferably comprise InP,InGaAs, or InGaAsP, and the waveguide may rest on an InP substrate.Other suitable materials may be equivalently employed for the substrateand/or the core, buffer, cladding, and/or etch-stop layers.

For a single-substrate vertical fabrication of a single- or dual-MLRdevice using InP/Al_(x)O_(y) MLR stack(s), a multi-quantum-well materialcomprising higher-bandgap InGaAsP barrier layers and lower bandgapInGaAsP quantum well layers may be employed as the electro-activematerial. This MQW material is lattice-compatible with the InP/AlInAsMLR stack(s) initially deposited, thereby enabling single-substratevertical fabrication. The wavelength range over which the usefulelectro-absorptive and/or electro-optic properties of this MQW materialmay extend is from about 1.2 μm to about 1.7 μm, which determines thepossible design wavelengths for the waveguide and the correspondingquarter-wave thicknesses for the MLR stack layers. The MQW material maycomprise the entire core layer (FIGS. 15-16 and 29-30), or may comprisea separate layer (FIGS. 17-18 and 27-28). Waveguides of thesecompositions are further processed by lateral oxidation of the AlInAslayers, as shown in FIGS. 44 and 46, thereby producing a high indexcontrast InP/Al_(x)O_(y) MLR stack (about 3.2 to about 1.5). In this andother cases where an entire quarter-wave layer is converted by lateraloxidation, the quarter-wave layer thickness for the initial materialdeposited must be determined based on the index of the final materialpresent after lateral processing (oxidation or otherwise).

For a single-substrate vertical fabrication of a single- or dual-MLRdevice using InP/Al_(x)O_(y) MLR stack(s), a multi-quantum-well materialcomprising higher-bandgap InGaAsP barrier layers and lower bandgapInGaAsP quantum well layers may be employed as the electro-activematerial in place of one or more layers of a MLR stack. This MQWmaterial is lattice-compatible with the InP/AlInAs MLR stack(s)initially deposited, thereby enabling single-substrate verticalfabrication. The wavelength range over which the usefulelectro-absorptive and/or electro-optic properties of this MQW materialmay extend is from about 1.2 μm to about 1.7 μm, which determines thepossible design wavelengths for the waveguide and the correspondingquarter-wave thicknesses for the MLR stack layers. The MQW material maycomprise one or more layers of the MLR stack(s) (FIGS. 19-20 and 31-34).Waveguides of these compositions are further processed by lateraloxidation of the AlInAs layers (if present), as shown in FIGS. 44 and46, thereby producing a high index contrast InP/Al_(x)O_(y) MLR stack(about 3.2 to about 1.5). In this and other cases where an entirequarter-wave layer is converted by lateral oxidation, the quarter-wavelayer thickness for the initial material deposited must be determinedbased on the index of the final material present after lateralprocessing (oxidation or otherwise).

A waveguide or resonator according to the present invention may includea MLR stack(s) comprising alternating quarter-wave layers ofAl_(x)In_(1-x)As (index of about 3.2) and Al_(y)O_(z) (index betweenabout 1.5 and 1.7). The MLR stack layers deposited during verticalfabrication (FIGS. 15-18 and 27-30) comprise alternating layers ofAl_(x)In_(1-x)As (quarter-wave thickness based on an index of about 3.2)and Al_(x′)In_(1-x′)As (quarter-wave thickness based on an index ofabout 1.6), with x<x′. The fraction x may range from about 0.8 to about0.9, while x′ may range between about 0.9 and about 1.0. Alternatively,AlAs/InAs super-lattice layers may be employed having relative AlAs andInAs sub-layer thicknesses yielding average Al/In fractions of x/1−x andx′/1−x′ for the initially deposited MLR layers. In either case (AlInAsor AlAs/InAs super-lattices), the aluminum fraction x of thelower-aluminum layers may vary with distance from the substrate,yielding a tapered waveguide structure. The electro-active layer maycomprise InGaAsP MQW materials as described hereinabove. Lateraloxidation of the waveguide proceeds more rapidly in theAl_(x′)In_(1-x)′As MLR layers having the higher Al content. The lateraloxidation is allowed to proceed just to completion in theAl_(x′)In_(1-x′)As MLR layers, thereby leaving medial portions ofAl_(x)In_(1-x)As between lateral Al_(y)O_(z) portions in theAl_(x)In_(1-x)As layers (FIGS. 48 and 50). The resulting DBR stackcomprises low-index quarter-wave aluminum oxide layers alternating withlayers having a high-index quarter-wave Al_(x)In_(1-x)As medial portionsurrounded by low-index aluminum oxide lateral portions. In the instancewhere the Al_(x)In_(1-x)As medial portion comprises a super-latticematerial, the sub-layers are typically sufficiently thin that foroptical wavelengths typically used in the waveguides and resonators ofthe present invention, the optical mode behaves substantially as if thesuper-lattice layer were a uniform layer having an index equal to theaverage index of the sub-layers of the super-lattice. A core layer maypreferably comprise InGaAsP electro-active MQW material, and doped InP,InGaAs, or InGaAsP may preferably be used for electrical contact layers.Either p-type of n-type doping may be used for the contact layers, anddelta doping may be preferred. Buffer, cladding, and/or etch-stoplayers, if present, may preferably comprise InP, InGaAs, or InGaAsP, andthe waveguide may rest on an InP substrate. Other suitable materials maybe equivalently employed for the substrate and/or the core, buffer,cladding, and/or etch-stop layers.

The GaAs-compatible MQW materials discussed previously (GaAs/AlGaAs;GaAs/InGaAs; GaAs/InGaAsN) may be used with InP-compatible MLR stack(s)using the multi-substrate vertical fabrication processes of FIGS. 21-26and 35-40 in similar manner to the use of InGaAsP MQW layers withGaAs/AlGaAs MLR stack(s) described above.

A specific layer sequence is given in the table in the Appendix for adual-DBR waveguide structure for the 1.5 μm region. The waveguide isvertically fabricated according to the three-substrate scheme of FIGS.35 and 36 (electro-active core on InP) and horizontally fabricatedaccording to FIG. 41, 42, or 43 and FIG. 48 or 50. Layer composition andrefractive index is given for the layers as initially deposited andafter lateral oxidation. Quarter-wave thicknesses are determined basedon the layer index after oxidation. The DBR stacks comprise alternatingAlGaAs/Al_(x)O_(y) layers and the electro-active core layer comprisesInGaAsP MQW material. This specific structure is exemplary only, andshould not be construed as limiting the scope of inventive conceptsdisclosed and/or claimed herein.

Any and all specific material combinations and operating wavelengthranges given here are exemplary, and should not be construed as limitingthe scope of inventive concepts disclosed and/or claimed herein. Inparticular, as new material combinations and systems are developed whichfacilitate enhanced material lattice compatibility and more extensiveoperating wavelength ranges, such materials may be employed inwaveguides and resonators of the present invention while remainingwithin the scope of inventive concepts disclosed and/or claimed herein.

Waveguides and resonators according to the present invention may findwide applicability in the field of fiber-optic telecommunications andmodulation and/or routing of optical signal power transmission. Suchresonators and waveguides may be readily incorporated into integratedoptical devices, and their unique optical properties enable operation atlower operating drive voltages than currently deployed devices, moreefficiently transfer of optical power to/from integrated opticaldevices, and/or lower insertion loss for optical devices. While thefollowing exemplary devices employ a fiber-optic tapertransverse-coupled to a multi-layer waveguide of the present invention,analogous devices may be equivalently implemented using other low-indextransmission optical waveguides transverse-coupled to the multi-layerwaveguide, including various fiber-optic waveguides, planar waveguidecircuit waveguides, and so forth.

An optical waveguide 5100 fabricated according to the present inventionon substrate 5110 is shown in FIG. 51 transverse-coupled to atransmission optical waveguide, in this example fiber-optic taper 5190.The waveguide/taper assembly is shown as surface-transverse-coupled inthe exemplary embodiment of FIG. 51, but fiber taper 5190 mayequivalently be side-transverse-coupled to waveguide 5100, and theensuing discussion applies to either transverse-coupling geometry. Modalindex matching may be adjusted for substantially negligible transfer ofoptical signal between fiber taper 5190 and waveguide 5100, therebyallowing optical signal to be transmitted substantially undisturbedthrough fiber taper 5190 and/or waveguide 5100. Alternatively, modalindex matching may be adjusted for substantially complete transfer ofoptical signal between fiber taper 5190 and waveguide 5100. This simpleconfiguration may be employed to provide a variety of optical deviceshaving low insertion loss. For example, the device of FIG. 51 may serveas an input coupler for efficiently transferring optical signal powerfrom an optical fiber to an optical device integrated onto substrate5110. The efficient optical signal power transfer enabled bytransverse-coupling yields a device exhibiting low insertion loss. Thedevice of FIG. 51 may be similarly employed as an output coupler forefficiently transferring optical signal power from an optical deviceintegrated onto substrate 5110 to an optical fiber. Waveguide 5100 maybe designed and fabricated for passive modal index matching tofiber-taper 5190 or other transmission optical waveguide. Alternatively,waveguide 5100 may include one or more active layers for enabling activecontrol of modal-index matching and optical signal power transfer(yielding an input/output coupler that may be turned on/off in responseto an applied control signal).

If waveguide 5100 includes an active layer, then application of acontrol signal enables control of the modal index of a guided opticalmode of waveguide 5100, in turn enabling control of the relative modalindex matching condition between the optical mode of waveguide 5100 anda propagating optical mode of optical fiber taper 5190 and opticalsignal power transfer therebetween. An electronic control signal may beemployed, for example, applied to an electro-active layer throughcontact electrodes 5120 and 5130, the electrodes typically including ametal film to enable application of control signals to contact layers inthe multi-layer waveguide structure. The waveguide/taper assembly maytherefore be used for altering optical signal transmission through fibertaper 5190, for example, and would potentially require substantiallylower control voltage due to the highly dispersive MLR stack. A deviceas shown in FIG. 51 may be used as a variable optical attenuator (VOA),with the level of attenuation varying with the amount of optical signalpower transferred out of the optical fiber and into the waveguide (whichin turn depends on the modal-index-matching condition resulting from acontrol voltage applied to electrodes 5120/5130). If electrodes5120/5130 are adapted for receiving high-speed signals, device 5100 mayfunction as a non-resonant high-speed modulator for an optical signalcarried by fiber-optic taper 5190. The device of FIG. 51 may also beused as a 2×2 optical switch, enabling controlled transfer (or not, asdesired) of optical signals between waveguide 5100 and fiber-optic taper5190.

Waveguide 5100 may alternatively include an electro-absorptive layer.Application of a control voltage through contact electrodes 5120 and5130 may enable control of optical loss in waveguide 5100, in turnenabling control of transmission of an optical signal throughfiber-optic taper 5190. Waveguide 5100 may alternatively include anon-linear-optical layer. Application of an optical signal may thereforeenable control of optical loss of waveguide 5100 and/ormodal-index-matching between waveguide 5100 and fiber-optic taper 5190,in turn enabling control of transmission of an optical signal throughfiber-optic taper 5190.

For this and subsequent embodiments of the present invention, someconsideration of the size and placement of contact electrodes isrequired. For a surface-guiding and/or surface-coupled waveguide, anupper electrode (such as 5130) preferably does not extend across theentire width of the upper surface of the waveguide, but is confinedalong one or both sides of the waveguide upper surface so as to reduceelectrode-induced optical loss for the surface-guided optical mode. Anupper contact layer of the waveguide structure (which may generallyintroduce less optical loss than a metal film) may preferably extendacross substantially the entire width of the waveguide for applying thebias voltage thereto through electrical contact with the electrode. Fora non-surface-guiding and side-coupled waveguide, a contact electrodemay extend across the top surface of a waveguide. Similarly, a lowercontact electrode (such as 5120) preferably does not extend under thewaveguide, but provides electrical contact with a lower contact layer ofthe waveguide which does extend under the waveguide across substantiallythe entire width thereof. The length of the electrodes should preferablybe chosen to result in the desired degree of optical signal powertransfer according to the equations shown hereinabove.

FIG. 52 shows a simple Mach-Zender interferometer modulator 5200,similar to the prior-art device of FIG. 1, fabricated according to thepresent invention on a substrate 5210 and including an electro-opticlayer. The optical signal to be modulated may enter modulator 5200through entrance face 5202 (end-coupling) and divide into a fractionentering the two branches of modulator 5200. Application of controlvoltages through contact electrodes 5220/5222/5230/5232 enable controlof the relative modal indices of optical signal fractions propagatingthrough the two branches of modulator 5200, in turn enabling the controlthe relative phase of the optical signal fractions at exit face 5204.When the fractions constructively interfere at 5204, the transmission ofmodulator 5200 is substantially maximal (except for insertion loss).When the fractions destructively interfere at 5204, the transmission ofmodulator 5200 is minimal (preferably nearly zero). The highlydispersive properties of the DBR stack(s) of modulator 5200 result in asubstantially lower V_(π) for modulator 5200 (less than 1 volt;potentially less than about 100 mV) than for the lithium niobatemodulator of FIG. 1 (as much as 5 to 10 volts). A high-speed driver foramplifying high-speed electronic control signals is therefore not neededto control modulator 5200. Modulator 5200 may be a single- or dual-MLRdevice. Modulator 5200 may also include an electro-absorptive layer,thereby enabling control of optical loss. This may be useful forcontrolling overall transmission, or for balancing intensities in thetwo branches of the interferometer for modulation contrast enhancement.Alternatively, modulator 5200 may include a non-linear-optical layer forenabling control of relative phase and/or optical loss by application ofan optical control signal.

While the device of FIG. 52 requires lower high-speed control voltage,the optical signal to be modulated must still enter through end face5202 and exit through end face 5204 (end-coupling), and modulator 5200therefore may exhibit relatively high insertion loss (as high as about12-15 dB; similar to the prior art device of FIG. 1). FIG. 53 shows aMach-Zender interferometer modulator 5300 fabricated according to thepresent invention as a waveguide structure on substrate 5310transverse-surface-coupled to a fiber-optic taper 5390 at an inputregion and an output region. The optical signal of fiber-taper 5390 maybe transferred to modulator 5300 by application of an input controlvoltage through contact electrodes 5320/5330 to impose the needed modalindex matching condition to achieve nearly complete transfer of opticalpower from fiber-taper 5390 to waveguide 5300. This input controlvoltage need not be modulated, and may therefore be adjusted to therequired level without the need for any high-speed driver. Once withinmodulator 5300, high-speed control voltages (or optical control signals)may be applied through contact electrodes 5322/5332/5324/5334 to controltransmission through modulator 5300 in a manner completely analogous tothat described hereinabove for modulator 5200. An output control voltage(which need not be modulated) applied through contact electrodes5326/5336 may be adjusted to achieve nearly complete transfer of anyoptical power transmitted through modulator 5300 back into fiber taper5390 in an output region of modulator 5300. This embodiment has thedesirable low V_(π) of the device of FIG. 52, but with extremely lowinsertion loss (less than about 3 dB, potentially even less than about 1dB). An optical detector integrated onto substrate 5310 at exit face5304 may serve as a useful diagnostic tool for monitoring theperformance of the device of FIG. 53. FIG. 54 shows a similarMach-Zender modulator waveguide 5300 side-transverse-coupled tofiber-optic waveguide 5390. If appropriately designed and sufficientlyaccurately fabricated, passive modal index matching may be employed atthe input and output regions, eliminating the need for electrodes5320/5330/5326/5336.

FIG. 58 illustrates an alternative Mach-Zender interferometer opticalmodulator according to the present invention. This device may be usedfor controlled modulation of light transmission through a taperedoptical fiber. A fiber-optic taper 5890 is showntransversely-surface-coupled to a waveguide 5800 at separate input andoutput coupling regions 5805 and 5806, respectively. The modal index ofwaveguide 5800 and the lengths of the coupling regions 5805 and 5806 maybe designed so that about half of the optical power is transferred fromwaveguide 5800 to fiber-optic taper 5890 at each of the regions 5805 and5806 without application of any bias voltage (passivemodal-index-matching). Alternatively, an input bias voltage may beapplied to the input coupling region 5805 (active modal-index-matching)through contact electrodes 5820/5830, each typically comprising a metalfilm to enable application of a bias voltage to contact layers in themulti-layer waveguide structure. The applied bias voltage is chosen totransfer about half of the optical signal power between waveguide 5800and fiber-optic taper 5890. The output coupling region 5806 may besimilarly passive modal-index-matched or active modal-index-matched (bybiasing contact electrodes 5824/5834). When employed, input and outputbias voltages applied are typically not substantially altered once theappropriate voltage levels are determined for a desired degree oftransverse optical coupling, therefore no high-speed driver electronicsare required for the input or output bias.

The intermediate segments (between the coupling regions) of thefiber-optic taper 5890 and the waveguide 5800 may function respectivelyas the two arms of a Mach-Zender interferometer, through which twofraction of the optical signal propagate. Application of a modulatorcontrol voltage through contact electrodes 5822/5832 enables control ofthe modal index of the modulator fraction of the optical signal inwaveguide 5800 in the intermediate segment thereof. Control of the modalindex in turn enables control of the relative phase of the modulatorfraction and the fiber-optic taper fraction of the optical signal asthey reach the output coupling region. The relative phase may beadjusted to achieve substantially constructive interference of theoptical signal fractions in the fiber-optic taper (i.e., maximaltransmission through the tapered optical fiber), or alternatively toachieve substantially destructive interference of the optical signalfractions in the fiber-optic-taper (i.e., minimal transmission throughthe tapered optical fiber), thereby achieving the desired result ofcontrolled modulation of the overall transmission of optical powerthrough fiber-optic taper 5890.

The modulator control voltage may be varied between two operationallevels corresponding respectively to maximal transmission (constructiveinterference in the fiber-optic taper) and minimal transmission(destructive interference in the fiber-optic taper). The differencebetween the operational voltage levels is V_(π), which may be less thanone volt (and potentially even less) for the modulator of FIG. 58incorporating a dispersion-engineered multi-layer waveguide according tothe present invention. The modulator may therefore be operated withoutthe need of a high voltage RF driver or amplifier, reducing the size,expense, and power requirements of the modulator, and eliminatingbandwidth restrictions potentially imposed by a driver. The insertionloss of the device may be quite low (less than 3 dB, perhaps less than 1dB), particularly compared to the end-coupled device of FIG. 1. Thespatial-mode-matching requirements and resulting insertion losses of themodulator of FIG. 1 are not present in the modulator of FIG. 58. Anoptical detector integrated onto substrate 5810 at exit face 5804 ofwaveguide 5800 may serve as a useful diagnostic tool for monitoring theperformance of the device of FIG. 58. Alternatively, waveguide 5800 maybe optically coupled at its input end face 5802 and/or its output endface 5804 to other optical elements, including optical sources and/oroptical detectors, integrated onto substrate 5810.

While shown as substantially identical structures in the Figures, theinput and output coupling regions of the Mach-Zender device need not besymmetric. Whether biased (active) or passive, each coupling region maybe specifically and separately configured depending on the operationalcharacteristics desired for a specific device. By appropriatelyfabricating, configuring, controlling, biasing, and/or adjusting theinput and/or output coupling regions, one or all of the average opticaltransmission level, transmission differential, the contrast ratio, thepower-off transmission state, and/or the power-on transmission state ofthe modulator may be varied, for example. Equivalently, one may setdesired minimum and maximum transmission levels for the modulator.Incorporation of an electro-absorptive layer in waveguide 5800 enablescontrol of overall optical loss of the modulator. Optical controlsignals may be employed for control of a waveguide 5800 incorporating anon-linear-optical layer.

An alternative Mach-Zender interferometer optical modulator according tothe present invention is shown in FIG. 59. In this case a fiber-optictaper 5990 is transversely-side-coupled to a waveguide 5900 according tothe present invention at input and output coupling regions 5905 and5906, respectively. Intermediate segments of waveguide 5900 andfiber-optic taper 5990 form respectively the two arms of a Mach-Zenderinterferometer. The modal index of waveguide 5900 and the lengths of thecoupling regions 5905 and 5906 may be designed so that about half of theoptical power is transferred between an optical mode of waveguide 5900and a propagating optical mode of fiber-optic taper 5990 at each of theregions 5905 and 5906 without application of any bias voltage (passivemodal index matching). Alternatively, an input bias voltage may beapplied via contact electrodes 5920/5930, and an output bias voltage maybe applied via contact electrodes 5924/5934, as described hereinabove(active modal index matching). A modulator control voltage may beapplied via contact electrodes 5922/5932. An optical detector integratedonto substrate 5910 at exit face 5904 of waveguide 5900 may serve as auseful diagnostic tool for monitoring the performance of the device ofFIG. 59. Alternatively, waveguide 5900 may be optically coupled at itsinput face 5902 and/or at its output face 5904 to other opticalelements, including optical sources and/or optical detectors, integratedonto substrate 5910. The operational characteristics and advantages,including low insertion loss and low V_(π), of the modulator of FIG. 59are similar to those of the modulator of FIG. 58.

Instead of operating as an electro-optic Mach-Zender interferometermodulator, the devices depicted in FIGS. 58 and 59 may be implemented aselectro-absorptive modulators. Waveguide 5800 or 5900 may be fabricatedwith at least one electro-absorptive layer thereof. The input and outputcoupling regions (5805 and 5806, or 5905 and 5906) may be biased orunbiased, and fabricated, configured, controlled, biased, and/orotherwise adjusted to provide substantially complete transfer of opticalpower between waveguide 5800 or 5900 and fiber-optic taper 5890 or 5990,respectively. Application of a control voltage to the intermediateportion of waveguide 5800 or 5900 through electrodes 5822/5832 or5922/5932 may alter the optical transmission through waveguide 5800 or5900 between substantially minimal and substantially maximaltransmission. In this way the overall transmission through fiber-optictaper 5890 or 5990 may be similarly modulated between substantiallyminimal and substantially maximal transmission. Such anelectro-absorptive modulator has low insertion loss (less than about 3dB) and would require a drive voltage comparable to currentelectro-absorptive modulators. It may be desirable to provide theintermediate portion of fiber-optic taper 5890/5990 with a optical lossmechanism, so that any optical signal not transferred to waveguide5800/5900 is not transmitted through taper 5890/5990.

Instead of modulating optical transmission through a tapered opticalfiber, the devices of FIGS. 58 and 59 may be used instead to modulateoptical transmission from the waveguide to the optical fiber as in FIGS.60 and 61. This may be particularly advantageous when a device of FIG.60 or FIG. 61 is combined with an optical source (preferably a diodelaser) integrated onto the same substrate as the waveguide and coupledinto the waveguide at an input end 5802 (FIG. 60) or 5902 (FIG. 61).Control voltages applied to input and output coupling regions may beemployed to control the overall transmission of light from the waveguideinto the tapered optical fiber (active modal-index-matching), or passivemodal-index-matching may be employed. Application of high-speed controlvoltages to the intermediate region of the waveguide enables high-speedmodulation of transmission of light from the waveguide into the taperedoptical fiber. When implemented with an integrated optical source suchas a diode laser, the devices of FIGS. 60 and 61 each solvesimultaneously the problems of: i) efficient coupling of light from thesource into an optical fiber; and ii) high-speed, low voltage modulationof transmission of light from the source through the fiber.

FIG. 55 shows a simple 2×2 switch 5500, similar to the prior-art deviceof FIG. 2, fabricated according to the present invention on a substrate5510. The optical signal to be controlled may enter coupler 5500 throughentrance face 5502 or 5503 (end-coupled). Application of controlvoltages through contact electrodes 5520/5522/5530/5532 enable controlof the relative modal indices of coupler optical modes propagatingthrough the coupling region of coupler 5500, in turn enabling thecontrol the relative optical power reaching exit faces 5504 and 5505.The highly dispersive properties of the MLR stack(s) of coupler 5500result in a substantially lower V₀ for coupler 5500 (less than 1 volt;potentially less than about 100 mV) than for the lithium niobate couplerof FIG. 2 (as much as 5 to 10 volts). A high-speed driver for amplifyinghigh-speed electronic control signals is therefore not needed to controlcoupler 5500. Coupler 5500 may be a single- or dual-MLR device. Switch5500 may also include an electro-absorptive layer, thereby enablingcontrol of optical loss. This may be useful for controlling overalltransmission, or for balancing intensities in the two arm of the switch.Alternatively, switch 5500 may include a non-linear-optical layer forenabling control of relative phase and/or optical loss by application ofan optical control signal.

While the device of FIG. 55 requires lower high-speed control voltage,the optical signal to be controlled must still enter through end faces5502 or 5503 and exit through end faces 5504 and 5505, and switch 5500therefore exhibits relatively high insertion loss (as high as about12-15 dB). FIG. 56 shows a 2×2 switch 5600 fabricated according to thepresent invention on substrate 5610, transverse-side-coupled to a firstfiber-optic taper 5690 at an input region and an output region, andtransverse-surface-coupled to a second fiber-optic taper 5692 at aninput region and an output region. The optical signal from fiber-taper5690 or 5692 may be transferred to coupler 5600 by application of acontrol voltage through contact electrodes 5620/5630 or 5621/5631,respectively, to impose the needed index matching condition to achievenearly complete transfer of optical power from one of the fiber-tapers5690 or 5692 to coupler 5600. This input control voltage need not bemodulated, and may therefore be adjusted to the required level withoutthe need for any high-speed driver. Once within coupler 5600, high-speedcontrol voltages may be applied through contact electrodes5622/5632/5623/5633 to control transfer of optical power within coupler5600 in a manner completely analogous to that described hereinabove forcoupler 5500. Output control voltages applied through contact electrodes5624/5634 and 5625/5635 may be adjusted to achieve nearly completetransfer of any optical power transmitted through coupler 5600 back intofiber tapers 5690 and/or 5692 in output regions of coupler 5600. Thisembodiment has the desirable low V₀ of the device of FIG. 55, but withextremely low insertion loss (less than about 3 dB, potentially evenless than about 1 dB). Optical detectors integrated onto substrate 5610at exit faces 5604 and 5605 may serve as a useful diagnostic tool formonitoring the performance of the device of FIG. 56. Switch 5600 may befabricated as a transverse-side-coupled device or atransverse-surface-coupled device, and both possibilities illustrated inFIG. 56. Fiber-taper 5690 is shown side-coupled to switch 5600, whilefiber-taper 5692 is shown surface-coupled to switch 5600. The twoseparate waveguides of switch 5600 are shown transverse-side-coupled inFIG. 56. If appropriately designed and sufficiently accuratelyfabricated, passive modal index matching may be employed at the inputand output regions, eliminating the need for electrodes5620/5630/5621/5631/5624/5634/5625/5635.

The embodiment of FIG. 51 may be configured to function as a 2×2 switch,with the fiber-optic taper 5190 and waveguide 5100 serving as the twooptical pathways of the switch. Application of a control voltage toelectrodes 5120/5130 (or application of an optical control signal)alters the modal-index matching condition between taper 5190 andwaveguide 5100, so that entering optical signals either remain withinthe component through which they entered (taper 5190 or waveguide 5100),or are transferred to the other component. The dispersive MLR structureof the waveguide enables switching at low V₀.

A resonant optical power control device similar to those described inearlier-cited applications A13 and A22 is shown in FIG. 57. An opticalresonator 5700 and a modulator waveguide 5702 are fabricated accordingto the present invention as transverse-side-coupled structures onsubstrate 5710. A fiber-taper 5790 is shown transverse-side-coupled toresonator 5700. A specific wavelength component of an optical signalpropagating through fiber-taper 5690, resonant with a resonance ofresonator 5700, may transfer into resonator 5700. A desired level ofoptical power transfer may be achieved through application of a controlvoltage through contact electrodes 5720/5730 to control modal indexmatching between the fiber-taper 5790 and the resonator 5700.Alternatively, passive modal-index matching may be employed. Theresonance frequency of the resonator 5700 may be controlled byapplication of a control voltage through contact electrodes 5722/5732,by changing a modal index of resonant optical mode of resonator 5700. Alevel of optical loss for resonator 5700 may be controlled byapplication of a control voltage through contact electrodes 5724/5734,by changing modal index matching conditions between resonator 5700 andwaveguide 5702 and/or by changing optical absorption characteristics ofwaveguide 5702. Changing the level of optical loss of resonator 5700 inthis way in turn enables controlled modulation of transmission ofresonant optical signals through fiber-taper 5790. Such devices, theiroperation, and their fabrication are disclosed in greater detail inearlier-cited application A7. Resonator 5700 and/or modulator waveguide5702 may alternatively include a non-linear-optical layer for enablingcontrol of resonant frequency and/or optical loss by application of anoptical control signal.

For purposes of the present disclosure and appended claims, theconjunction “or” is to be construed inclusively (e.g., “a dog or a cat”would be interpreted as “a dog, or a cat, or both”), unless: i) it isexplicitly stated otherwise, e.g., by use of “either . . . or”, “onlyone of . . . ”, or similar language; or ii) two or more of the listedalternatives are mutually exclusive within the particular context, inwhich case “or” would encompass only those combinations involvingnon-mutually-exclusive alternatives. The present invention has been setforth in the forms of its preferred and alternative embodiments. It isnevertheless intended that modifications to the disclosed active opticalwaveguides and resonators, and methods of fabrication and use thereof,may be made without departing from inventive concepts disclosed and/orclaimed herein. composition before composition after thickness (nm)lateral oxidation oxidation substrate n/a GaAs GaAs contact 100 n-deltadoped n-delta doped InGaAs InGaAs buffer 500-1000 nm GaAs GaAs DBR stack230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateralAl_(x)O_(y) medial Al_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)AsAl_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medialAl_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) 130Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medial Al_(0.94)Ga_(0.06)As230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateralAl_(x)O_(y) medial Al_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)AsAl_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medialAl_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) MQW 50 InGaAsP1.2 μm InGaAsP EO bandgap 1.2 μm bandgap core 9 InGaAsP 1.6 μm InGaAsPbandgap 1.6 μm bandgap 20 InGaAsP 1.2 μm InGaAsP bandgap 1.6 μm bandgap9 InGaAsP 1.6 μm InGaAsP bandgap 1.6 μm bandgap 20 InGaAsP 1.2 μmInGaAsP bandgap 1.6 μm bandgap 9 InGaAsP 1.6 μm InGaAsP bandgap 1.6 μmbandgap 20 InGaAsP 1.2 μm InGaAsP bandgap 1.6 μm bandgap 9 InGaAsP 1.6μm InGaAsP bandgap 1.6 μm bandgap 20 InGaAsP 1.2 μm InGaAsP bandgap 1.6μm bandgap 9 InGaAsP 1.6 μm InGaAsP bandgap 1.6 μm bandgap 50 InGaAsP1.2 μm InGaAsP bandgap 1.6 μm bandgap DBR stack 230 Al_(0.98)Ga_(0.02)AsAl_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medialAl_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) 130Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medial Al_(0.94)Ga_(0.06)As230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateralAl_(x)O_(y) medial Al_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)AsAl_(x)O_(y) 130 Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medialAl_(0.94)Ga_(0.06)As 230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) 130Al_(0.94)Ga_(0.06)As bilateral Al_(x)O_(y) medial Al_(0.94)Ga_(0.06)As230 Al_(0.98)Ga_(0.02)As Al_(x)O_(y) contact 50 n-delta doped n-deltadoped InGaAs InGaAs buffer/clad 200 nm GaAs GaAs

1. A method for fabricating a multi-layer laterally-confineddispersion-engineered optical waveguide structure, comprising the stepsof: depositing a layer structure on a substrate, the layer structureincluding a multi-layer reflector stack and an active layer; andspatially-selectively processing at least a portion of the multi-layerreflector stack or the active layer so as to provide lateral confinementfor a guided optical mode.
 2. The method of claim 1, further includingthe step of processing at least one side of the multi-layer waveguidestructure to provide at least one layer of the multi-layer waveguidestructure with at least one lateral lower-index portion.
 3. The methodof claim 2, the lateral lower-index portion being provided by oxidationof a lateral portion of the layer.
 4. A method for fabricating amulti-layer laterally-confined dispersion-engineered optical waveguidestructure, comprising the steps of: depositing a first layer structureon a first substrate, the first layer structure including a multi-layerreflector stack; depositing a second layer structure on a secondsubstrate, the second layer structure including an active layer;securedly positioning the second substrate relative to the firstsubstrate so as to substantially eliminate voids between the first andsecond layer structures; removing the second substrate while leaving theat least a portion of the second layer structure; andspatially-selectively processing at least a portion of the first layerstructure or the second layer structure so as to provide lateralconfinement for a guided optical mode.
 5. The method of claim 4, furtherincluding the step of processing at least one side of the multi-layerwaveguide structure to provide at least one layer thereof with at leastone lateral lower-index portion thereof.
 6. The method of claim 5, thelateral lower-index portion being provided by oxidation of a portion ofthe layer.
 7. A method for fabricating a multi-layer laterally-confineddispersion-engineered optical waveguide structure, comprising the stepsof: depositing a layer structure on a substrate, the layer structureincluding a first multi-layer reflector stack, a second multi-layerreflector stack, a core layer therebetween, and an active layer; andspatially-selectively processing the first multi-layer reflector stack,the second multi-layer-reflector stack, the core layer, or the activelayer, thereby providing lateral confinement for a guided optical mode.8. The method of claim 7, further including the step of processing atleast one side of the multi-layer waveguide structure to provide atleast one layer thereof with at least one lateral lower-index portionthereof.
 9. The method of claim 8, the lateral lower-index portion beingprovided by oxidation of a portion of the layer.
 10. A method forfabricating a multi-layer laterally-confined dispersion-engineeredoptical waveguide structure, comprising the steps of: depositing a firstlayer structure on a first substrate, the first layer structureincluding a first multi-layer reflector stack; depositing a second layerstructure on a second substrate, the second layer structure including asecond multi-layer reflector stack, the first layer structure or thesecond layer structure including a core layer, the first layer structureor the second layer structure including an active layer; securedlypositioning the second substrate relative to the first substrate so asto substantially eliminate voids between the first and second layerstructures and so as to position the core layer between the first andsecond multi-layer reflector stacks; removing the first substrate or thesecond substrate while leaving at least a portion of each of the firstmulti-layer reflector stack, the core, the second multi-layer reflectorstack, and the active layer; and spatially-selectively processing thefirst multi-layer reflector stack, the core layer, the secondmulti-layer reflector stack, or the active layer, thereby providinglateral confinement for a guided optical mode.
 11. The method of claim10, further including the step of processing at least one side of themulti-layer waveguide structure to provide at least one layer thereofwith at least one lateral lower-index portion thereof.
 12. The method ofclaim 11, the lateral lower-index portion being provided by oxidation ofa portion of the layer.
 13. A method for fabricating a multi-layerlaterally-confined dispersion-engineered optical waveguide structure ona substrate, comprising the steps of: depositing a first layer structureon a first substrate, the first layer structure including a firstmulti-layer reflector stack; depositing a second layer structure on asecond substrate, the second layer structure including a secondmulti-layer reflector stack; depositing third layer structure on a thirdsubstrate, the third layer structure including an active layer, thefirst layer structure, the second layer structure, or the third layerstructure including a core layer; securedly positioning the thirdsubstrate relative to the first substrate so as to substantiallyeliminate voids between the first and third layer structures; removingthe third substrate while leaving at least a portion of the activelayer; securedly positioning the second substrate relative to the firstsubstrate so as to substantially eliminate voids between the second andthird layer structures and so as to position the core layer between thefirst and second multi-layer reflector stacks; removing the secondsubstrate while leaving at least a portion of the second multi-layerreflector stack; and spatially-selectively processing the firstmulti-layer reflector stack, the core layer, the second multi-layerreflector stack, or the active layer, thereby providing lateralconfinement for a guided optical mode.
 14. The method of claim 13,further including the step of processing at least one side of themulti-layer waveguide structure to provide at least one layer thereofwith at least one lateral lower-index portion thereof.
 15. The method ofclaim 14, the lateral lower-index portion being provided by oxidation ofa portion of the layer.